If only it were that easy...
- Mandatory software updates can change UIs that run on """your""" machines.
- I still remember when I was running Debian Unstable, the day I ran a routine upgrade which replaced GNOME 2 with GNOME 3, I couldn't find my way around, and couldn't even easily revert because Linux distributions ship all their software in synchronized packages with interdependencies, and aren't built around picking and choosing older package versions (especially foundational packages).