I understand your point with SMP on your existing PPC. But going true SMP breaks compatibility, it's as simple as that.
I am really curious. Why on earth are we still trying to have compatibility with the dead old classic Amiga software? MorphOS working on a powerful 64bit AMD multicore system with seamless Amiga emulation integrated to the OS sounds like a perfect deal to me, rather than imprisoning ourselves in the 90's just to have compatibility with seriously old and mostly useless software.
I am an Amiga user for 30 years, but isn't it time to move on?