To begin with, remember that the new kernel certainly won't be able to be ported to 68k - it definitely wouldn't be OS4. And Amigans would certainly be laughed at if the new OS (or even part of it) would run on ancient 68k processors.
The world has not stood still - but the Amiga has. IMO we simply cannot expect a new OS to run on early 90's hardware. "Classic" Amigas are falling to bits, and we need a clean break from the past. Of course, you can still upgrade them to PPC to run it - what other new OS would you upgrade from 60 to 200Mhz to use it :-) Perhaps the naming of OS3.9 was deliberate - aside from a recurring decimal, nothing else could have indicated better that it was the last ever 3.x OS.
Besides, there are other issues - I doubt that Hyperion would have the resources to co-ordinate and support OS3.9999 or whatever.