I'm sceptical with Windows, considering that most programs are installed via EXE files, so the outcry will be huge. But I'm not saying it can't be a possibility.

With Ubuntu there would only be a chance of it happening if they also make their distro immutable. That way the user could not as easily install packages the traditional way. But even then there might be ways to disable this immutable mode for troubleshooting. However, this, in my opinion, would cause a mass exodus as Canonical does not have the same advantage as Microsoft or Google have: Windows and android are, to an extent, closed off ecosystems. Thus switching to another system is very hard, as not every software is available on every other system, so potentially subpar alternatives and comparability layers, whose functionality mostly depends on whether the company behind the original system is actively fighting against these tools or not. Ubuntu on the other hand, is a Linux distro, so you cab make it like Theseus and recreate this distro more or less with the sum of its parts, if need be.

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.