The dream: buy anywhere, play seamlessly with anyone, the game is the hub rather than the platform.
The reality: you end up logging into 8 different publisher 'friend'/library platforms if you are currently playing 8 multiplayer games. You open a laptop and load some launcher or website and import friends lists, and fiddle with shit to try to get playing with your friend on another platform. Now I guess these publisher accounts are already happening to a greater and lesser extent on console, but it's mostly an optional extra on some console games, for detailed stats or mobile companion apps.
I kinda want cross-play I kinda want Sony to allow it, it would be neat to play with the one person I know on Xbox. But the reality is messier, more complicated, and uglier than the dream. I don't really see any real benefits of it so far on PS4/PC cross-play, or anything else. Maybe devs get an increased matchmaking pool, but that's not really an end-user benefit, unless they like some sparsely populated game.
So if this actually happens one day and Sony relinquish control, you aren't just going to use the controller to jump across to friends on PS4, see BillyXbox is currently playing Halo, invite him, party up, and have a Call of Duty party. No. It's going to be weird launcher or website on a laptop, sign up with third party game account, import friends list, check third party launcher or load game to check they are online, blah blah.
Fortnite cross-play experience
Between PS4 and PC, it works, if you have linked steam to epic games launcher, tied your PS4 account to it, and dicked about. To be honest I found literally just playing against other steam friends difficult, because we both had to import steam contacts to epic launcher. And of course, playing against PC people with 144hz monitors and mice when on PS4 is kinda pointless, but I'm sure PC players enjoy the reverse. I've tried this with different people a few times and it's had mixed success, some of my friends just literally give up at the setup phase. Meh
Rocket league cross-play experience
It's coming, supposedly. Right now it's just rocket league cross-platform matchmaking pool, and to be honest, I turn it off on PS4 and it makes zero difference to the experience. I do use it on PC though, it's a smaller player pool so it helps get games quickly on PC. Supposedly cross-platform-parties are coming this summer, but I guarantee it's similar to Fortnite, complicated, fiddly, requiring me to dick about on a laptop to add contacts to some launcher or website.
I guess I just hope we don't end up with a complicated un-curated mess in the name of cross-play. As long as it all remains optional for the people who want to take advantage of it.