I assume that was an answer to my third point, but I think you may have misunderstood me there. To take an example, if I use Compiz as a window manager, with wobbly windows enabled, most of the time my windows will not be wobbling. If they wobbled while I typed into them, that would annoy me greatly. At this point in time, there is little visible difference between Compiz and Metacity - all windows are shown front on, at 100% with no rotation or anything. And if I were using the Mesa software rasterer instead of an accelerated graphics card (yes, I know that Compiz refuses to start in that case, but that is not my point), it would not be very difficult for the rasterer to spot that the onscreen tiles (my windows) were identical before and after Compiz had finished transforming them. So the transformation can be reduced to a simple blit, which can be handled perfectly well by legacy cards.
So in effect, what I was meaning was that it ought to be possible for Mutter/Gnome shell to use OpenGL as a backend and still work fine on legacy systems in most situations. And with a bit of smart optimisation (setting the level of effects, handling the effects cleverly) it should even be possible for them to work reasonable well on legacy hardware at all times.