“Quid pro quo” might or might not be a basis for impeachment. It would depend on what the “quid” and “quo” were in a given situation. The reason that what president Trump did IS impeachable is because he asked president Zelensky to help him gain a personal advantage over a political rival in return for US military aid during a wartime for Ukraine. The military aid was authorized by Congress and a US president cannot attach a personal gain to it. I strongly believe president Trump does not understand this distinction, but his telephone call was definitely NOT “perfect.” It may be true that in other circumstances that some kinds of “quid pro quo” are perfectly ethical when the country (US) and not the individual is the beneficiary of the ask.