I suspect you could coax 720p out of a 1GHz+ G4 if the code was sufficiently optimized -- another obvious problem with putting the video decoder in a black box.
But more importantly, the point is not the exact quantity of particular models of PowerMac in existence, the point is that if I have one which is otherwise capable of it, this nonsense interferes with me doing it.
And the same goes for every other thing that doesn't fit a mold. What about Linux on a PS3? What if I have a PA-RISC/Itanium/Power/etc Unix workstation? What if I'm using X forwarding so that my "browser" is really running on an UltraSPARC server?
It breaks anything the central planning committee didn't contemplate a priori or can't be bothered to fix. And it's not just old hardware, it's anything new or different. It keeps new platforms from getting off the ground.
I think you might be too optimistic on this one.
Not to mention most of these are probably running with 512Mb to 2GB RAM (G5 iMacs go to 2GB, PowerMacs to 8GB, some go to 16GB)