It'll be interesting to see what system becomes the defacto standard. They're too similar for many people to want both. I suspect that Playstation did so well this generation that the PS5 will win by default. They're still the go to for Japanese developers so they get a ton of exclusives just by virtue of that fact. Microsoft are going to have to get way more exclusives than Forza, Halo etc. to bring in new users.
People apparently talk about backwards compatibility being a big deal but very few people actually use it. If you want PS3 and PS2/1 compatibility you can always buy the consoles they're plentiful and cheap as chips.
The 360 stomped the PS3 during its respective generation, because Sony was just as out of touch as, or more than, M$ was at the launch of this most recent generation. The fan bases are fickle. Mess with DRM, disregard online capabilities, etc, you'll have a lot of pitchforks and torches.
I imagine you're right, tp. It isn't in any developer's best interest to release their games on a single platform, and no others, especially when the hardware differences are so minimal. There's usually a lot of money, at a bare minimum, involved.