Honestly, I think most people running such old hardware wouldn't ever need to update X anyway. What benefits would you get from doing so? None of those chips can run a modern DE, and old versions of X are fine for minimalistic window-management.
As has been said before, XAA doesn't actually accelerate anything anyone is using on a desktop anymore (Bresenham lines and onscreen solid fills are more typical of graphical systems from the 80's and early 90's), so chances are you'd get better performance on these machines using a shadowfb (which has also been said before). I really doubt anyone who has been updating their software for the last few years will miss XAA at all, and *most* hardware capable of supporting EXA in a meaningful way already does. As for driver breakage, I imagine most newbie coders could track down all the XAA stuff in the driver and nuke it so it'll compile for a new server without acceleration.