So at least a single person has to read a book for a movie to be made.
Yes, because movies aren't made on POPULAR* books...
* If you think it takes only a single person for someone to decide that making a movie would be profitable, then you're being fabulously optimistic here.
That proves what, exactly? The fact that TV and film adaptations are always far more popular than the books they're based on says it all.
If both literate and illiterate people can watch a movie, then obviously the audience for movies will be potentially bigger. Hell, people don't have to be illiterate, just not having the time or the desire to read a book without some encouragement. I know some cases when a movie (or a game) made people pick up a book it was based on. It doesn't change the fact that the usual trend is as follows: the book becomes popular -> somebody makes an adaptation of it. Look at Game of Thrones.
Disco Elysium is trite like all your examples of literature, so no surprise it would have some chance of success compared to something that requires a moderately high reading level, but it's still not very popular and is a niche game compared to RPGs of the past that were mainstream for their time.
Bullshit. By conventional reasoning Disco Elysium should've been a failure (even I thought it would be). It isn't. And it's EXTREMELY popular for what it is. Popular enough to get a TV adaptation. That's not something usual, especially if - as you claim - you consider it to be "not very popular" and "niche" product. By the way, compare sales of Disco Elysium to "mainstream RPGs of the past". I am really curious how will the numbers look.
Also, lul at "Disco Elysium is trite". It has original setting (not my cup of tea, but I can at least give credit where credit is due). It goes back to RPG roots when it comes to character-player dynamic. It is written well enough for a plenty of people to actually enjoy reading over a million words.
Popularity is only a finance consideration. Plenty of people turn down popular things.
Not plenty enough to not make it popular apparently. Because if it's popular, then it means it is "liked, admired, or enjoyed by many people". Enough to make the whole model sustainable.