Canonical doesn't force anyone to use anything. Snap is open source, just modify it to use a different store if you want. Mint literally forked a zombie DE, but changing a few lines of code in snap is an issue...
Defaults matter a lot, snap is not open source (client is, backend isn't), you cannot "just modify it (Ubuntu)" to use a different store, because Ubuntu installs snaps even with apt. Mint is not part of the discussion.
