That's it. Besides, that PPC Macs are "useless" is just a subjective evaluation or opinion that the vast majority of people on this particular forum will happily disagree with. No big deal as everyone is entitled to his/her own opinion in my opinion.
IMO no computer is truly "useless". Every computer ever made, will likely still be able to do most of the same things now, that it could do back when it was new. A PowerBook G4 now, can still watch video/DVD, edit photos, play music, mix audio, write documents, create presentations, and browse Wikipedia, just as well now as it could back then.
What has changed, is that newer computers are better at doing that stuff now, it's not that older computers have gotten any worse at doing those jobs, it's just that newer computers have gotten better/faster.
Sadly, there are some things that have/will get worse on the old computers, thanks to the internet. Since newer computers are getting faster, websites get more bloated to "take advantage of" these faster computers, this of course, makes the browsing experience on older computers, much worse(since they can't handle these huge Javascript-based websites).
Lastly, there is also a software component. For example, with 16GB of RAM, a 1TB SSD, and a 2TB 7200RPM HDD, and a Quadro FX 4500, a Quad G5 should still be able to handle almost everything a modern PC or modern Mac can. But, it can't because no one writes new software that will work on it(OS X 10.5 was the last PowerPC supported MacOS version, and no one writes apps that support 10.5 anymore let-alone PowerPC-optimized 10.5 apps. Linux is just as bad, because although some distros still support PowerPC, and have new apps, almost none of them are optimized for PowerPC so they run slower than they really should.)