There are a lot of stakeholders involved and a lot of applications relying on DNS may have problems (browsers). So the migration path is long and winded I guess.

Other than that, no. GNS itself could also be realised on top of other DHTs, such as libp2p/IPFS (before anyone mentioned the, eh, maturity issues of the reference implementation in GNUnet.)

You can't shim GNS into looking like DNS to the OS?

You can. Either using nsswitch plugins or a local DNS service that resolved the names in GNS. But, browsers are delicate when it comes to TLS certificates, for example. See also https://www.ietf.org/archive/id/draft-schanzen-gns-19.html#n...

There's no technical reason it couldn't, but a big non-technical one: it doesn't allow for the rent-seeking behavior that supports ICANN and the internet name industry.

But would it be necessary for ICANN to participate in or consent to someone else (trustworthy) making their registry available over GNS?