> What non-blockchain solution solves the double spend problem when transferring digital assets in a peer-to-peer network?
Literally any trusted central authority or database.
> Or, in the case of Ethereum, providing solutions to general-purpose decentralized computation and state (rather than only peer-to-peer payments) with such strong public consensus?
You haven't actually stated a problem here, you've described a solution in search of a problem.
> User A holds digital asset X (such as a valuable domain name "xyz.eth")
You're mentioning a .eth domain name being bought with cryptocurrency as an example, which is entirely circular. "Hurr durr, betcha can't swap one blockchain thing (.eth domain) for another blockchain thing (cryptocurrency tokens) without using a blockchain" isn't as strong an argument as you think it is. If we were talking about a .com domain name, no blockchain in the world will help you with that transaction.