>Nintendo would be notably absent from any of the platforms and only allow their ROMs to be streamed from their own service.

They already do this, you need a Switch online subscription to access the NES/SNES ROMs they have available

Oh, I know, but in the theoretical world where there's a Netflix like subscription, I would assume that means a lot of different IP was also gathered there, like Sega, Atari, older Playstation and Xbox titles, etc.

There's not a lot of incentive for some of those groups to come together, but I imagine even if most could be assembled, Nintendo would be particularly resistant.