Really stupid question here. I don't pretend to know a whole lot about the PS/Xbox universes but are we sure that Activision will stop releasing content on the PS5? I mean, Microsoft acquired Minecraft but the games still came to the Wii U and the Switch. Banjo Kazooie and Goldeneye (rumored) are also making appearances again on Nintendo consoles. For MS to ban Activision from releasing content on the PS5 when the PS5 comprises a huge chunk of yearly CoD sales seems like a questionable business decision even if MS and Sony are direct hardware competitors.
Well some analysts in the gaming industry have speculated that the acquisition will be difficult to go through if they keep Call of Duty away from PlayStation, and Phil Spencer just recently tweeted about having calls with leaders from Sony and their desire to keep Call of Duty games on PlayStation.
So they're discussing deals about that.
I don't know about the legality issues of all this, but I imagine most of the big titles would not be coming to PlayStation, if Microsoft can make it so.
I think a big goal for MS is to have Sony accept GamePass on PlayStation, and moves like this are probably their best bet into making that a possibility.