I don’t really understand Canonical’s decisions. Early on Ubuntu really did make things simpler, but now days most distros have caught up in terms of usability, and now it just seems like Canonical tries to do things differently for no benefit. (See trying Unity, but switching back to Gnome, or trying Mir, but switching back to Wayland.)
Yeah, it’s gotta be a replacement. You can’t typically mix-and-match UI toolkits.
Since flutter is just a canvas drawer you can mix-and-match flutter with other UI toolkits (assuming someone created such a bridge)