The problem has been the lack of chipsets.
In a way, you could even say that AGP put bullet holes in many of the efforts to resuscitate the Amiga 199x-through-yesterday. They took a long time to arrive and there were never a plethora of such on the market.
Now that PCIe will be the "one true bus" for a while, if PowerPC-land can survive the next few years it should be easier to design systems that don't have to come out completely obsolete.