Earlier quoted context omitted.
It's just as easy for a game developer to say "eh we're no longer using that blockchain for game assets" or otherwise make your "purchase" (which isn't really a purchase) unusable with a blockchain based solution as it is with current Steam/iOS-based systems. (Probably easier, really.)
You're missing the point, the game developer obviously has to buy into the ecosystem, but the difference is with the current status quo in-app purchases are not really owned by the customers, so even if another game developer wanted to honour their purchases there is no way to interop. Web3 is a pretty good use case for this, and the transaction costs barely add anything if you compare to the cut the platform takes (…
Roblox already does this. Your in-game purchases are yours and you can carry them from one game to another.
Platforms outcompete protocols - this is why web2 became centralized.