I don’t think it would be viable in the sense that we can only change Ubuntu Linux so much before it stops being Ubuntu Linux. That said, yes, it would be (and is) viable to continue expanding Ubuntu Core until it gets fancier and includes more of the platform. Not as a replacement, but as a complement. For example, I really enjoy the idea of being able to develop my snaps in Ubuntu proper and then having a super lightweight platform in Ubuntu Core to put these snaps live elsewhere.
Note that in Ubuntu Core we have a snap named “classic” which brings on board APT and other things into the environment. It’s a bit the opposite of snapd in a classic distribution: instead of being snaps into the classic world, it brings the classic world into a snap platform.
PS. I’ve changed your post slightly to refer to Ubuntu Core, which is its actual name. Sorry if I got it wrong, and please feel free to tweak it further.