Assuming the rendering engine is swapped out for an OpenGL one (doable), there are issues with MFC, plugins and plugin architecture (most plugins also depend on MFC/Win32 and there are 70+ of them) and you'd need a write-once compile-twice setup for each plugin to stop yourself going absolutely mad. The IDE would be more complicated because it depends on a Windows-only UI library, but I don't think that needs to be worried about before the runtime.
In short, time is the limiting factor - as said earlier in the thread, we're all volunteers, full-time students working in our spare time. To be honest, I'm not a big fan of spending months of time and effort on an operating system I don't use to gain another 5% of the PC market. Someone else is welcome to do it though if they want - we are open source - and I'll gladly answer any questions or help any way I can if someone took up the challenge.