I think it's perfectly reasonable not to offer the UK special treatment over other nations. It would certainly have been more generous to allow UK registrants to retain their domains, but it doesn't seem to me more reasonable. The EU's responsibility is to its members, and they negotiated the withdrawal agreement accordingly.

The UK could, of course, have made access to the .eu TLD a higher priority when negotiating the withdrawal agreement. I expect we could have secured it. I don't see any reason to believe that the EU witheld it unreasonably.

I don't think it's mud-slinging to point out that the EU didn't kick us out!

Could have, but shouldn't.