Just last year, two Call of Duty games made it to the list of best-selling PlayStation games. It should not come as a surprise that Sony knows that Call of Duty is a big deal. In fact, Sony is referring to Call of Duty as "an essential game: a blockbuster, an AAA-type game that has no rival."
The statement comes after Brazil's regulatory body questioned Sony about its thoughts regarding Microsoft's acquisition of Activision Blizzard. In its answers, Sony doesn't hesitate to emphasize the importance of Call of Duty to the PlayStation brand.
Sony claims that "Call of Duty is so popular that it influences users' choice of console." Sony adds that the best-selling FPS franchise has no rival because "no other developer can devote the same level of resources and expertise in game development."
Sony had more to say about Call of Duty and how it continues to sell well despite "down" years. But, the gist of it is that Sony seems worried, as it should be.
Sony and Activision are long-time partners. Specifically, Call of Duty has always had special perks for PlayStation owners. This applies to the upcoming installment, Modern Warfare 2. However, the partnership between the two will soon end. Even if Microsoft will honor the existing agreements between Activision and Sony, the deals will only last for two more Call of Duty games after Modern Warfare 2 comes out. More importantly, there's no guarantee that Call of Duty will be available on the PlayStation years from now.
You can argue all you want about how much Microsoft and Activision could potentially lose in sales, but that's a risk that the tech giant is financially prepared to make.
Keep in mind that The Elder Scrolls 6 and Starfield would "probably" sell better if they came out on both the PS5 and Xbox Series S/X, but they're not. Instead, Starfield will be exclusive to the Xbox and PC platforms. This prompted an apology from Bethesda. Call of Duty might sell far more units on an annual basis than most video games, but the point remains.
TLDR; There's nothing that Sony can do if Microsoft buys Activision and wants to keep Call of Duty all to itself.
If it's any consolation, Sony does have an extensive library of exclusives that it could port to the PC platform to help soften the blow if it loses Call of Duty. After revealing that it expects to make at least $300 million from PC ports by March, Sony has dropped info on a handful of upcoming PC ports.
The first one to come is Marvel's Spider-Man on August 12. The rest, such as Uncharted: Legacy of Thieves Collection and The Last of Us Part 1, have no release dates yet. Finally, there's plenty of hard proof online that PC ports of Returnal and Sackboy: A Big Adventure are coming as well.