If he is not in, too bad. If you want to keep Unity going, you (or the team) have to learn how to maintain/develop Compiz. If Compiz fails, Unity would die. What is the window manager of Ubuntu 16.04 or 17.04?
Nemo or Nautilus is not the main problem. There are so many other file managers around. The thing is to find out how that DE was created, whether you (the team) can create it from scratch (mc4man).