MorphOS team is very small and everything that is been done today should be done in a way that makes sure you don't have to use those small resources later on to fix the shortcomings of that 1st effort.
ASMP,SMP or whatever will only be useful if apps get coded for the new APIs and these apps should continue to work if and when the next step towards a modern OS is taken.
While it is true that no MorphOS app runs out of !!31Bit!! address space that is because they are restricted by those 31Bit. Wayfarer for sure could utilise more than the 1.8GB available on G5 and as such an "NG" API should be 64Bit to ensure forward compatibility for any code written with that API.