Sure you could say it about “any language,” but I think you’re skipping a lot of nuance with your examples: python has notoriously had a long transition from 2 -> 3. C is 40+ years old, and the semantics and idioms of the language aren’t changing from month to month.
I think the parent comment is making the point that the pace of change and evolving idioms/features of Rust means that code you write today may need to be updated in a far shorter timespan than the typical timeline for working code (a few months, rather than several years). The bitrot is just a lot faster right now than other languages.
Considering the implications of relying on an external company as the registry, I don’t think custom domains are really “vanity” as much as reserving agency to move the code if it becomes necessary. I’m perfectly happy with GitHub, but would rather my modules didn’t break if they implement a policy change at some future date. I also don’t like the implication that “GitHub owns” my repo due to the import path stuff.
That being said, I wonder if this same thing could be achieved with a simple reverse proxy/CDN with a few rewrite rules? Ideally, the only cost to a typical maintainer would be the domain name, and the rest could be hosted on free infrastructure (cloudflare would seem like a reasonable choice).