That can be said about almost every merger or acquisition. You even get media and telcom companies that get approved for consolidation with probably zero benefits to competition and consumers yet they go through. If these kinds of deals get approved then any can - especially for something non essential like video games.
Sure new things can happen. With MS, Activision/Blizzard games can blow out to all kinds of devices, cloud, GP, even Samsung TVs (with GP) for cheap. You might not even need a dedicated gaming device to play COD or Diablo, nor need to pay $70. That's a great benefit.
COD isnt even a big portion in overall industry sales. Also, even as part of Sony's gaming division sales, COD (from what someone posted one time as a ballpark portion) was tiny. If someone wants to dig up that detail or do new math, I think Sony's gaming division does $12 billion sales at around $3 billion profit per year. There is no way Activision games (even if all are added up) does a big portion of their $$$ per year.
The fear is that if COD disappears from PS 3 years from now when the current partnership ends, MS cuts the cord in 2026 (unknown if true), that shit loads of PS gamers bail ship for Xbox or PC, so the value of COD (+ Diablo etc..) is worth more than it's actual sales. That Activision is worth more than the sum of their parts as some gigantic halo effect on the PS gamer even though most PS gamers dont even play COD or Diablo.
This will be impossible to prove in any meaningful way.