Earlier quoted context omitted.
The EU continues to control .eu because it is available only to EU citizens. Nothing changed. Your surprise exactly reflects the typical beliefs of a Brexit supporter, you thought the idea is you get to keep all the benefits you had before, and also you get rid of any downsides you didn't like, and somehow it's the EU's job to help you achieve this after you leave. That didn't make any sense in 2016, and it still did…
But this policy doesn't make sense for the EU either. They lose domain registration revenue. Thousands companies migrating to a new domain name will lead to confusion for EU citizens too, etc. It just seems like a case of "we want it to be as painful as possible for you, even if we have to take a bit of additional pain for us too".
If I somehow owned tax.gov, and was running a tax prep service from it, I doubt the “I’m grandfathered” argument would fly.
It’s likely similar to lawyers.eu (I made that domain up), etc; people should be able to expect that to be an EU based service, not something overseas.