This reaction comes up a lot in these conversations, with a more or less explicit critique of authors for not picking the medium best suited to their vision. I’d just like to pop up a note that from my own taste and perspective as a reader, IF with a set MC can work very well as IF, not as a failed novel.
The aspect of IF I enjoy most is exploration, and fiction offers lots of different things to explore. Variable plots and subplots; different possibilities for character development and growth; a rich and interesting setting (which as I’ve written before is I think an underpraised pleasure of good literature).
IF with a set MC can still offer me that chance to explore all of those things. Horizon Zero Dawn, one of my favorite AAA games of the past few years, had a terrifically rich story and setting. A CoG that let me explore a similarly narrative-rich world, even without graphics, would be well worth my time. Aloy doesn’t change much as a character, she’s just set loose on the world to explore (and becomes more herself as she does so, intensifying characteristics that she clearly had from the beginning), but discovering it through her distinct perspective as a marginalized outcast moving to the heart of power is part of the fun.
And when it comes to exploring character development and growth… that can work with a set MC too, often even better than with a customizable MC. Geralt in Witcher 3 is a character you can explore and (to a meaningful extent) develop through your choices in game. With lots of IF, there’s nothing of the MC for a reader to explore or learn because it’s all in their hands; which can make it hard for the author to write an arc that emerges satisfyingly from the character’s past. Often custom MCs end up just being a viewpoint from which to explore the plot and setting, rather than themselves being a character worth digging into.
And that can work great! I don’t at all think a customizable MC is bad, as you could see from my own IF work so far. MC customization (like second-person perspective) can be a great authorial tactic to get readers fully engaged with the story and world – and in the context of the wider gaming world, it has the additional benefit of “not othering” readers who until recently have had few opportunities to play protagonists who are like them. I just also think that a great IF can be written with a set MC.
In a case where the protagonist is set, and the plot is on hard rails, and the setting isn’t one I’m particularly excited to run around digging into (even if it’s otherwise a perfectly fine setting)… then, sure, I’d have the “why isn’t this a novel” reaction. But if any of the above are genuinely explorable, I’ll understand why it was written as IF.