It doesn't matter as much as you think. It isn't about competing with one game, but every game by every publisher across the entire industry. For Activision Blizzard's games on console (not just COD, but everything) accounted for 10% or less of global revenue & global market share. It's the same on PC as well. It's only through the combination of Microsoft and Activision Blizzard that the two of them together would be between 10%-20% of global market share and revenue on consoles.
EA actually has a larger market share worldwide for their titles in 2021 than even Activision Blizzard does. These are numbers provided directly by Microsoft and the other publishers CADE reached out to.
This deal, though this seems to have all kinds of attention right now, is as straightforward as it gets. There's no way based on the hard data in a phase 2 investigation CMA can possibly arrive at a likely significant lessening of competition based on the 'balance of probabilities' legal standard.
Also, COD could lose the title this year to Elden Ring.

But we shall see.