Oh, and, yes, it's true that Py_RETURN_NONE currently takes advantage of Py_INCREF being an rvalue, and changing Py_INCREF to a statement would break the existing implementation. But Py_RETURN_NONE itself is of necessity a statement. We would simply change Py_RETURN_NONE's implementation to multiple statements, probably with the do { ... } while(0) trick, so it worked again. I'd be shocked if that change broke any existing code. So that's no big deal.
Having external code that depends on Py_INCREF being an rvalue is my concern, and what I hoped you'd bring up on bug #17206.