I mean... my friend had a PS3 for years then eventually bought an Xbox 360 because he wanted to play COD with the rest of us. Then he decided to get halo and gears etc that he couldn't play before.
Were there cross play, he never would have bought the 360 and the exclusive games. Then he started buying most stuff on Xbox because he had invested in Gold and everything so might as well use it.
That's why I wouldn't do it if I was Sony, especially when most people are playing them multi platform stuff on your system. Why give them the opportunity to say "hey I actually like the Xbox! I should just get stuff here now since I can still play with my PS4 friends".
I'd personally love it, having bought several games on both Xbone and PS4, but I get why they don't.