A triple A game isn't viable on PC alone nowadays, it has to release on console and hence has to account for what modern (both PC and console) players expect. You're claiming to be a fan of this niche but haven't played any of the major releases in it from the past 10 years (Dishonored 1 & 2 + Prey), if you haven't played those games as a fan of the genre then what chance does the genre have? These were big games, marketed well by a major publisher (Bethesda).
As for genres, your point about only x number of games becoming a success is taken, but I guess my counter to that is, if you become a success in the shooter genre the upside is 10x or 100x what it would be for an immersive sim because the market for fps is much much bigger than those for immersive sims.