I think it's likely to be regularly released titles that are in danger of slipping under the radar already, especially franchises that are losing steam. For example, I think fighting games that aren't Street Fighter or Mortal Kombat have little chance of AAA success nowadays. I think annual sports titles that aren't FIFA, Madden, or NBA 2K are slipping down there (e.g. UFC/MMA, PGA golf, WWE wrestling, skateboarding, etc.). You are correct about RTS games falling off - genre-adjacent games like MOBAs and Tower Defense games have fallen off alongside them. I think that shoot-em-ups, brawlers, and platformers fell down there a long time ago. I think simulation games like flight simulators are on their way into the sunset. I think short-form narrative games (e.g. Uncharted) have also become unsustainable outside of special circumstances like being platform exclusives.
Basically, AAA budgets can't exist if enough people aren't actively interested in the type of game and willing to keep playing it for a long time. It's quite normal for genres to mature and a handful of established industry leaders to carve out a sustainable audience and everybody else to lose ground until they basically fall out completely.
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