While, sure, 2D games have become a bit of a niche, I'm not really sure how something that uses 3 buttons and a stick is less accessible than something that needs 12 buttons, 2 sticks, gyro and a d-pad. Do you mean 'unusual' rather than 'inaccessible'? Because to most people just starting out with computer games, 2D games are much easier to play. Spelunky especially starts off with 'move right and jump', which is much easier to get your head around than spinning a clickable joystick and controlling a camera in a 3D space at the same time for a rookie gamer.
As for AAA titles, sure, Assassins Creed or CoD is mainstream, but not all of them are not 'niche'. Something like X-com, or RPGs in general, require a damn sight more effort to get your head around if you don't usually play them than something like Resogun or Velocity.
Personally Ps+ would be completely useless to me if they just loaded it up with sports games and cinematic AAA stuff, that would ignore most of the variety that exists outside of such narrow definitions, but I agree with any argument that says a mix would be best. To me that means a minority of AAA stuff due to it making up a relatively small amount of the games released, despite it's high visibility and popularity. Obviously zero AAA stuff isn't good either.