I see nothing convenient in having 2 programming languages serving exactly the same purpose. And the entire community knowledge would split, making harder to find information for “your” language. And now some people would write plugins in C++, some in C#.
A language is just a tool.
The long compilation time in somebody’s project isn’t directly caused by used language. It depends more on build tools provided with the engine.
Unity only recently introduced proper incremental compilation for C#, previously small projects could compile for minutes every time. And launching Play In Editor is incredibly slow in Unity, partially because resetting scripting states (they’re fixing it finally).
Unreal Build Tool only rebuilds what gets in the dependency chain of changed code files. You can have a huge project and you won’t notice slower compilation while the project grows. Big codebase won’t slow down editor operations.
Also now we have Live++ integrated, so you can recompile changes in .cpp even while the game is running.
Yep, C# syntax is much cleaner and it’s initially unpleasant to switch to the older language, but t’s not a reason to overcomplicate the entire engine.