It's quite a considerable saving. I suppose by "fix UX" you mean make it so the saving would be less anyway, but I think really they're just conceptually different:
- branch: pointer to a line of history, i.e. a commit and inherently its ancestors
- stash: a single commit-like dump of patches
If stashing disappeared from git tomorrow, I think I'd use orphan commits rather than branches to replace it.
In other words, shouldn't git just fix ux for branches and rip out stash?