Common sense is not that common, they say. We are just having a good example, quite possibly... ;^>
I think that modern G4s (not to say G5s) are quite overpowered for a vast majority of users who are using them for mereley typing in their documents, surfing the web and the like, provided they are not using M$ word and stuff (for some strange reason), which is absolutely bloated and work as a dog, and gets only slower each time...
The OS kernel would most probably not have much greater requirements than the current one, the interface, on the other hand, can and probably will be more and more resource hungry. But there is a solution to that one: the vast majority of games lets you select detaliazation level and graphics quality. Leopard will offer such feature AFAIK, so I can't see any problem here.
Multiplatform support in a UNIX environment is not a big deal too if everything is written well and let's hope it is.
Platform specific parts are typically those kernel parts written in the assembly language but they tend to be small and almost never change for a well established architecture (read: they exist for G4/G5). The rest is probably plain C/C++ or whatever that ports or should port flawlessly, it's gcc in the end of the day...Personally, I think that dropping PPC support would suck big time and be far too M$oftish... ;(
