> maybe they can change the PPC CPU to [...] G4's, or [...] G5 CPU's.

That would require a complete redesign of the board, which is further complicated by the fact that both (non-SoC) G4 and G5 need a northbridge chip to make them work. That aside, the QorIQ chip has been chosen due to its on-chip I/O capabilities (SATA, USB, Ethernet), effectively turning it into some kind of southbridge chip to the m68k chip. Neither G4 nor G5 can provide that.