It's worse than that - PPCs come with a terrific price premium over other non x86 processors.
Take a look at SBC and dev boards for PPC, MIPS, ARM or even Coldfire. To get to G4 class speeds typically costs you double or even triple what all those other arches cost to get you there...
The last chance for other cpu makers to really make significant inroads into the mainstream desktop market was in the early 90's. After that really the market was pretty much sewn up.
Yes, and the scary thing is the G4 is a pretty slow processor. Not much memory bandwidth and a single core. There are AMD processors on the market that sell for less than $50 that would slap the G4 silly.
And no one making PPC processors is going to tout their products as competitive in the PC market. The fastest PPCs are made by IBM and they're aimed at markets above the PC market, while all other PPC manufacturers shoot for embedded apps below the PC range.
While IBM could make a PPC or Cell/Xenon related processor that was at least close to competitive with X86 processors they flatly are not interested.
Trust me, I talked to the head of the division focused on Cell development. They only want to market their IP to companies they select for applications they approve of (read that as they feel they now where they can make money, and it ain't in PCs). Maybe their right, after all IBM designed processors are now a part of all three major game consoles.
Still, if a large company with deep pockets approached them and could guarantee they would purchase X# (read millions) of processors, they would listen. In that regard, IBM has my respect. They know how to remain profitable.