> it's all about the heap!

This. If you only use local (i.e., stack) variables, you should be able to directly transform your code into FP using a state monad. Hence I do not think there is a large difference between the two paradigms here.

Plus: Functional Code is easier to reason about. At least for the person that does the formalization. You simply need fewer constructs (and less complicated ones). The same argument holds for parallel code, btw.