MUI "5" is a non-issue, as MUI 4/5 is completely useless outside of MorphOS. It was used to compile Odyssey for OS4, but that project seems dead now. Other than that, no software outside of MorphOS (and very little MorphOS software) actually requires it.
If Jens and Thore want to continue working on it (and urinate all over other peoples' copyright with every release) - fine, it's their spare time. The rest of us only need to worry about it once YAM - Jens' and Thore's other project - starts requiring it. But since YAM's open source, that could be easily fixed (provided there's still somebody left who cares, once that happens).