> You can tell it's not property because the one thing that's guaranteed to result in losing a domain is failing to pay the fees.
There's a sibling comment that points out that courts recognize it as property, and certainly trademarks are a similar "property" that requires active enforcement.
But while the law is a good authority because they've gone through many disputes and have had to work out good arguments, I don't think it's the final authority; laws can change after all.
That a domain requires upkeep doesn't make it not property. Even in the absence of taxes, your house or any of your stuff requires some degree of upkeep.
But a domain is certainly not chattel, and intangible property always does seem like... not property. (Though, even with tangible property, it feels fuzzy, and that's part of why fences are used to reify borders.)
To my mind, a bigger issue with calling it property is that there's not necessarily a single registry system.
My thought experiment is to ask what we'd do without registrars. We'd all simply advertise our domains to the DNS servers, with all the obvious conflicts that registration is meant to avoid. And we'd have to resolve those conflicts by having the DNS providers agree that a particular advertisment was correct. I think that gets a bit closer to the heart of what "owning a domain name" means.
Owning an agreement with these entities to manage those disputes looks very similar to any other kind of security or bond. It also has qualities of an asset: you can trade it, it's not very liquid, you can derive an income from it by developing things on it, etc. That's why I lean on the side of the "domains are property" camp.