Like most greek letters, it’s overused. tau. Pi is also used to mean the prime number counting function. There have been papers that use pi in both contexts in short order. On zeros of cubic L-functions, for instance.
might be conceptually the wrong number, but we’re stuck with it, and is not what we want to replace it by. Why?
The nome is not going away, and if we really are committed to finding some new symbol for , then we need to commit ourselves to changing a whole swath of mathematical literature at once, which we may not be able to do easily.