That's right, Blizzard does cross-game unlocks already, through their own platform. I don't expect that companies like Activision/EA/Valve would be the ones to use NFTs for this kind of thing, since it's better for them to build and operate their own marketplaces, which gives them total control.
I completely agree with you on the boiling of the oceans. It's a problem that cryptos will have to solve in the long-term to be viable as any part of a green economy. There are a few projects that are energy efficient, but Ethereum and others have work to do. In any case, NFTs aren't specific to any one blockchain.
I think content creation and limitation is a separate issue, and not implicitly part of NFTs. You can issue a token representing a particular game object, with no ability for the player to customize or modify it. You bring up an interesting point though - there could be a limited, controllable level of customization available to players. Consider a smart contract that mints NFTs and allows certain attributes to be modified, which could affect the cosmetic or functional details of the in-game item they represent. You could sell a class of NFTs representing a certain sword, and players could pay extra for a golden version, or a version with infinite durability.
For Steam Workshop, I think you've identified exactly the issue. They control the marketplace, and set the rates. If it's more profitable for a developer to set their own rates/royalties, and it's easy enough to implement, and there are enough players willing to use NFTs (or a platform that manages NFTs for them) then we'll start to see developers use them. Marketplaces aren't mutually exclusive, so you could have some items on Steam, and others on blockchain. You could make certain desirable items exclusive to the platform that nets you the most profit.
I agree regarding difficulty of implementation - this is something that most developers won't bother with at the moment since it doesn't make economic sense, but as the ecosystem develops we'll likely see companies that provide a simpler abstraction layer for NFTs (perhaps even specializing in games). As the implementation costs drop, we'll start seeing adoption. This might take several years.
Regarding blocking I/O, I don't think it would work like that, just like it wouldn't work as p2p. A game server could poll the history of its smart contracts (or the abstraction layer provided by a service platform) every few seconds or minutes, and update its centralized database of items to match whatever transactions have occurred. All reads within the game would still be to its own database, and not directly from a blockchain.