Okay, let's pretend that's true.

Are the complications due to null protection being there, or are they due some other things?

Some of the complications are necessary. You either need generics or a whole bunch of primitives to have non-nullable references. Go would also have to replace slices with something different. Return types and the shape of the interfaces would have to be changed. Algebraic datatypes or some complicated way of defining return types would need to be added for it to be usable. Defer statements that can modify the function's return value would need to be handleable by the type system, or dropped.