PPC on classic Amiga is quite expensive way of running ppc datatypes under ClassicOS. It is $800+ mp3 player or slow .jpg viewer
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Yes, I know there is a Quake PPC...
I could get new PPC board for my classic for about $300 
I'm not quite sure what you mean by the last line (is it missing an 'if'?), but yes, I agree that its a way to expensive way to expand an Amiga.
One thing I do like about the fpga custom 68K approach is that the cpu doesn't have to be a perfect copy of the 68K cpu (especially the '040 and '60). So of the weirder incompatibilities (like the Zorro III bus and the '040s use of burst) can be ironed out.
And fpgas could always lead to faster ASICs.
PPC are better suited to OS4, MorphOS, Linux and other alternatives.
And I can run quake (or better games) on those systems, or PCs.
I like having alternatives, which is why I don't mind tweaking the sensibilities of the traditionalists.
Its kind of like my being a registered Republican, but not being anywhere near dumb enough to vote for Trump.