That's the problem, though: if they wanted to allow this, it's easy to do — for example, GOG has a Steam import feature but relatively few publishers allow their content to be used that way. Spending an enormous amount of money switching to NFTs would just be a distraction from the business reasons why those publishers are blocking that capability.

If you are living under the assumption that they would honour ownership, you don't need NFTs in the first place. The whole thing can be much more efficiently implemented without them.