Ubuntu is the fork; just use Debian.

There's very little incentive to switch. When people release for Linux they almost always first/only test it works on Ubuntu - hence it's going to be the least buggy. (ex: GOG only seemingly tested games on Ubuntu)

So the switching cost is buggy software. You'd need something radically different that brings enough new features to the table to make it worth it the switch and dealing with bad/non-existant support.

I think something like Nix/Guix but with stable library versions that matches Ubuntu/Debian LTS ones 1-to-1 could get traction

There are plenty of Ubuntu forks?