It won't adapt to new windows or directx versions, it won't be able to make use of new hardware, it won't have any new features the competition will come up with,... In a few years it will be useless and you will have an outdated skill set. If you don't just do 3D as a hobby then I wouldn't wait so long for a switch.

How can plugin authors fix bugs in the closed source core?

What a waste of time to work on plugins which will result in a patched up mess. :p

Milkshape doesn't progress either (actually I would call it dead too) but at least its author is still around for bug fixes as far as I know. If you are into simplicistic and limited 3D modeling and animation software then there are better alternatives like fragMOTION.