Take away Altivec from a G4 and your have, at least in the early versions, basically a G3 processor. In their day, they were considered one of the most advanced and powerful processors. These chips haven't gotten any slower, just our perspective has changed. I'm typing this on a Pismo G3 400. Not my fastest portable, but more than adequate for the job.
I have read in other threads how some lament how you tube seems to be the de facto litmus test as to a processors power and I have to agree with them. Even My 333 G3 lombard can play back dvd video, with the pcmcia dvd playback card. You tube is merely one corner of the web. Yes streaming content is the fastest growing part of the web, But every tool has it's use. I don't take a honda civic 4X4ing. I use a 4 wheel drive vehicle made for the job. But that does not mean that the civic isn't a useful tool.
Some users have Powerpc computers as that's what they can afford. Some are hobbiest who like pushing older technology to it's limits. Some, can afford newer, but choose for one reason or another, not to throw away money on the latest and greatest because they realize that they just don't require it. I've owned two Macbooks and sold them both. For me they weren't necessary and for some reason just did not feel like a Mac. I enjoy using my Powerpc Macs more including my G3's and I use my Pismo every day. I also still enjoy using 68k Macs as well.
Will I have to upgrade someday? Not unless Powerpc units disappear completely (If you can't tell, I don't really give a flip about you tube. Most of it is such a waste of time and resources.) Sorry for the long winded post.