>For variable sized arrays, there is already a bit of overhead in sizing the allocation. Would it really have been impossible to move large allocations to the heap with automatic free at exit of the scope?

Not a compiler writer here so I'm not sure. I think it wouldn't be too difficult, but then this goes very much against the exact control you normally expect from C. And violating expectations like this is usually not good.

It would also produce bigger functions in a somewhat opaque manner, because you would need the code to deal with malloc and free and stack allocation at the same time.