75% of the units sold or 75% of the overall revemue. Given that the most successful PC games aren’t even on Steam, the latter seems unlikely to me. Roblox alone is a sustained revenue stream in insanely high numbers.
Do they block the competition in any way? They aren’t the stewards of Windows. Epic buys exclusive rights to games. Does Valve do the same? On Steam Deck, there’s even an entire independent app store (Discover with Flathub) enabled right out of the box. That’s how the community made Minecraft and Heroic Game Launcher available. Official EGS, GamePass, and GOG launchers could be made available via Flathub but MS etc. choose not to.