I can't think of a single piece of MorphOS software that is both un-maintained and must have.
Well, the entire classic version is unmaintained AFAIK, but as we're talking about MorphOS 2.x...
Isn't backwards compatibility with (OS friendly) 68K, PowerUP and WarpOS applications one of the key features of MorphOS? If all that were dropped, what tangible benefits would it have over AROS?
So PPC emulation is not absolutely necessary as a few endian tweaks to the source and a re-compile would be the most effort needed for most software.
I think you are over simplifying it somewhat. A lot of effort was put into optimising areas of the code for the PPC vector unit and while I'm sure there are generic versions of all these, porting the optimised versions to SSE+ is certainly no simple compiler switch change. Just look at the apple developer information pertinent to this.