And to be able to fully utilize the engines capabilities, according to these use cases, source is a valuable asset.
Everything you say that you can do without source in Unity, could probably be done better with source.
To remake things, in stead of improving on the things that are already there, is counter-productive and in most cases a bad move, performance-wise. This is the daily life of a Unity-developer: To remake things, already in the engine, because what is there is incomplete or lacking. Pathfinding and GUI are good examples of this…
If you could improve directly on the engine source, you would also be able to program in c++ and circumvent the whole mono-package. This could solve many problems for unity devlopers, where mono is seriously outdated.