Domains are, sure but the space is much much larger. With namespaces the space is for example namespace x crate instead of just crate. With domains it's impossibly large. And if you introduce some kind verification with domains e.g putting something in the DNS TEXT entry, or calling a well-known path, or or or, you basically can't squat anything well-known anymore.

As for github, you must have a github account to create a crate on crates.io. to my knowledge there is no alternative crates registry. Point me to one if you do, because I'd be grateful and willing to publish my crates there.