mscorlib.dll & System.dll

Question

Why did MS originally make the decision to maintain these two separate core libs? Maybe they had some scalability issue in mind, but nowadays I never see an application, of

Answer 1

Expanding on Scott's answer.

Any given version of the CLR is highly tied to a particular version of mscorlib.dll. It is a special DLL in very many ways. The CLR runtime requires certain types/methods be available and implements many methods defined in the actual code base. The complexity of managing this relationship is reduced by having an unbreakable link between a CLR version, and version of mscorlib.