Yep - I can see the long term benefits but it requires pretty much a rewrite of Unity at this point, when a dll will solve the issue in the meantime. It's a good suggestion and probably one Unity's thought about many times. The UI for instance is open source. And I'm still not sure it actually stops people from doing anything given there is dll support. It just means less bloat for those who would have to be capable of dealing with dlls to begin with, or at least that level of competence. In any case a good suggestion for Unity 6 or such.