It seems, there are, in theory, 5 options for fallback/compatibility:
1. Make g-s and mutter scalable down to envs without 3d
2. Provide full compat/fallback mode, with panel and applets
3. Provide restricted fallback mode, only gnome-panel, just enough to do smth
4. Same as #3, just very basic and simple panel, as William mentioned
5. No fallback at all.
Anything I missed? Was that choice discussed anywhere?
Sergey