There's a lot of space between "buying nothing" and "buying the biggest game developer in the world" that Microsoft could have explored if they felt they needed to acquire more studios. You know, like when they bought Bethesda for $8 billion and had no trouble. But chose not to repeat that, and went for the throat instead.

There's only a handful of remaining game developers/publisher so systemically significant that a platform owner buying them should get serious antitrust scrutiny. (Activision, EA, Take Two, Ubisoft.)