In theory Canonical could lock down Ubuntu like that, but it would be the end of Ubuntu. Switching over to Mint or Debian is not a big deal for majority of the linux-users and also Ubuntu would lose all the advantages they can currently pull off from Debian package maintainers. Also I suppose it would bring a ton of headaches with licenses, but IANAL, so don’t quote me on that. And, obviously, that would kill snapcraft too as I don’t see any incentives for developers to support walled gardens for free, so it wouldn’t be all bad.