While it is a gray area, at the same time I don’t think it’s a difficult one. Deep down, you’re going to know if you’re taking too much. If you have to ask if it’s too close to what inspired you, odds are, it is.
And if anyone is unsure why some writers are reacting vehemently to this idea, feel free to ask a combat veteran how they feel about folks who engage in stolen valor. It’s a rather dramatic example but still a valid one.
I hope I won’t be too off-topic here, but I’m not sure that I really agree with you that so many authors blatantly ripping off Tolkien did some good and I definitely don’t think that it was a good thing and the (commercial) upsides weighed out or even equalled the downsides.
Firstly, as @Havenstone showed, there were other succesful fantasy works published at around the same time as the first Shannara book that were much less derivative of Tolkien, but still were commercially succesful. And we don’t know whether there wouldn’t eventually have been other, more original and less obviously Tolkien-inspired fantasy works that would have both caught the attention of the public, become hugely commercially succesful and started a new trend if the first Shannara book had never been published. It could very well be that this would eventually have happened with another, much better written fantasy book that caught the zeitgeist the same way that the first Shannara book did.
And even though the Shannara books may have helped Fantasy books became more popular with book-buyers, it also lead to an artistic decline in the genre, with the genre(at least outside of humorous fantasy and fantasy for teens or children) dominated by various Tolkien light series until sometime in the 90’s. The group of adventurers on quest to find a way to defeat the evil overlord(or its closest equivalent) trying to take over world-plot repeated ad nauseam, and with a few notable exceptions, such as Tad Williams, with little to nothing of what actually made Tolkien great and interesting in the first place or other ways to at least to come close to level of artistry, storytelling and world building of Tolkien. Though not all of the fantasy of that period were rehashing that formula, the majority, or at least the majority that we hear about, seemed to be. And it wasn’t until A Song of Ice and Fire and Robin Hobb’s Farseer Trilogy in the 90’s that fantasy finally had major influential books that stepped fully out of the shadows of Tolkien and his imitators( I consider Jordan to be a transitional figure). All of this focus on Tolkien also has lead to a disappointingly large portion of the fantasy fandom not really knowing much about the fantasy before Brooks, outside of Tolkien and maybe Howard. Writers like Leiber, Moorcock and Zelazny(with his Amber books) are talked about far too rarely, at least judging from the fantasy subreddit and writers from even before that, like Lord Dunsany, generally get mentioned even less.
Part of the problem with the Brooks influence, I think, was that people often seemed to be as influenced by the inferior imitator as the original. But I think another problem was that it lead to a massive case of “follow the leader” where lots of writers and companies hoping to equal or even better the success of Brooks respectively wrote and published stuff in the same vein to the point where that became a trend and a formula that dominated fantasy and left less space for fantasy books that were different from that and made fantasy books as a whole feel too “samey”. And in the same way, I think, if some concepts and formulas become too popular in COGs or HGs, there is a risk of so many COGs and HGs using them that COGs and HGs in general start feeling to samey. I certainly don’t think we are there yet, but if the influence of The Wayhaven Chronicles continues to grow to the point where most HGs reuse a lot of the concepts of that series, this can certainly lead to HGs feeling more samey instead of being as charmingly varied as they currently are. I’m not saying that I mind more HGs with lots of personality stats and where the MC discovers a world of supernaturals accompanied by four or so ROs, but if eventually every other or almost every HGs should eventually become like that, I would start to long for more HGs that take a different approach. And in the same way, though I don’t mind the slightly more complex visual novel without visuals approach of The Wayhaven Chronicles, I think it would be sad if that should become the standard HG approach. I greatly enjoy HGs and COGs with a more RPG-style approach, such as most of Lucid’s HGs. I also greatly enjoy HGs and COGs, such as the Evertree Saga, that combines deep romances and a reasonobly complex story with rpg-style features. And; last but not not least; I really enjoy HGs and COGs such as Jolly Good-cakes and ale, Life of a Wizard and Choice of Magics that get really branchy or where even making a few slightly different decisions can make for a really different playthrough. So I hope that neither The Wayhaven Chronicles or any other succesful HG(or for that matter COG) releases will have too much of a “follow the leader”-effect, to the point of “everyone” trying to make a HG or COG like that.
And if there should be more instances of writers coming to an agreement with writers of abandoned WIPs of continuing their stories or reusing concepts and ideas from those WIPs, I hope that at least a large portion of that will be with stories using really original concepts and ideas, because it would, at least to me, be much sadder to see those kind of stories, ideas and concepts disappearing into the ether, so to speak, than stories leaning into more “stock” ideas and concepts. Then again, when it comes to writing such stories without agreement from the writer who originally came up with them, I can see that reusing those ideas and concepts without consent from the writer who originally came up with them could much more easily slide into infringement theory. So that’s another reason why I hope that in such cases the writers who are unable to finish their stories can come to agreement with another person who can, without showing disrespect to the original vision.
@Havenstone and others – Is there enough interest to start a discussion about the fantasy works of the 1960’s, 1970’s, and the 1980’s?
I see numerous people dismissing 30-50 years worth of writing as “Tolkien-lite” or “nothing but rote imitators”. I have a very different opinion, and I know others do too.
On the other hand, I am not certain how valuable such a topic would be for this community.
Either way, I do not think this thread is the place to have that discussion.
To clarify, I didn’t bring up Brooks’ supporters because I agree with them, it was more of a, “Isn’t it fascinating that even in a case as blatant as his, there is still intelligent debate? We humans will never agree on bright lines, will we?” But I got distracted before I actually managed to make that point. I haven’t read Brooks or Tolkien (aside from The Children of Hurin, which my mega-Tolkien-fan friends picked for our book club — I couldn’t really get into it). I don’t actually have an opinion on his legacy or the amount of shame he should be experiencing. 
To get back to the “where is the line?” discussion, I think one of the difficult parts is insofar as we can even agree there is a line, the line is often only clear in retrospect. At the start of a new genre, it’s hard to predict which elements of the progenitor will be viewed as essential to the genre, staples of the genre, and differentiating features. And then there’s the initial lack of language to talk about the new genre, leading to progenitor comparisons, no matter how different you try to make it. Like how until the FPS genre became established, every FPS was a “DOOM-clone”.
In practical terms, I’d say the less-established the genre you want to add to is, the more you should err on the side of radical shake-ups.
On the note of “same premise”, I think you’d first need to define what you mean by that though… I just realized that Minecrat Story Mode and Heroes of Myth kinda have the same premise (or at least how I understand that word).
I’ll head over to the fantasy thread and continue the discussion thread there. But before I do that I just want to make clear that I don’t think there were 30-50 years of mainly “Tolkien-lite” fantasy. I’d say there were 15-20 years when Tolkien imitators dominated the field of fantasy, but I’m aware that there were also good stuff being made that wasn’t as indebted to Tolkien. And I do think there’s been lots of good fantasy books written and published since the mid 90-s. That’s it for me on this subject in this thread, I’ll continue with the discussion in the fantasy thread.