Which is actually exactly what the PPC only Amiga OS4 currently is. There are so few PPC powered Amiga's that there is little point in it from a marketing or commercial standpoint. It is only the continuing Amiga fan base that has any kind of interest in it.
And those of us without PPC are completely locked out from it, with PPC cards being so rare now. Therefore what is the difference between the current OS4 running on PPC or x86 hardware?
Moving the future development of Amiga OS over to x86 hardware would instantly open it's accessibility to nearly everyone on the planet. You can build a pretty good PC these days for less than some of the PPC accelerators for the Amiga have been selling for second hand lately!
As for software compatibility. What native PPC software is actually available for OS4? Not much. So what would the difference be? Dual core x86 CPUs are now pretty much the standard, so an extra core could easily be dedicated to transparent emulation for older software support. Therefore opening up a x86 native Amiga OS to backwards compatibility. Plus you could run UAE on it for classic Amiga support if needed (where the custom chip emulation is needed).
I don't think continuing along any development that needs proprietary hardware is a good idea. Although moving to x86 would open up the ability to use the CatWeasel controller for direct access to Amiga floppies.
The other advantage of x86 is that current processors are far ahead of PPC in development and their speed is much faster. And again, look at the technology and hardware available for x86 based systems. Compare that with that available for PPC powered Amigas. Graphics cards many years out of date, and limited support for other cards.
If they continue down the PPC route the Amiga OS will just fall further and further backwards into complete obscurity and eventually vanish.
Why create a new version of Amiga OS for x86? For the future, not the past! We have classic Amigas in our collections, as well as emulation for the past. Amiga fans want a new Amiga OS, running on cutting edge technology again, taking full advantage of the hardware and software created that is in the present.