X11 has a strong reference implementation hence there should be no need for a fallback. If you really would want to launch a program which needs the new protocol on an old/outdated server for whatever reason you could use a wrapper like Xephyr/Xnest as fallback. (Besides, Xorg already has many optional extensions and if one of those extensions is not present the program simply fails to launch. The fallback problem has not been a real problem so far because X11 development is very centralized.)