If Canonical really wants to use Debian only for the base distro and install everything else from Snap they're welcome to it as far as I'm concerned. Lots of people do exactly that albeit with Flatpak. I also do it on my server but with Docker.
Once they've fully migrated to Snap it's not impossible they might look elsewhere for the base. Once .deb compatibility stops being a factor they could use anything.
A Debian base can be an advantage for Ubuntu Server but they could also use different bases for different purposes, like gaming or RT multimedia.
And of course they could be looking into making the bases immutable images.