While you’re right that there’s nothing illegal or impeachable about the US spying on a foreign government, running this type of op on a formerly [emphasis added] close ally used to be considered a piss-poor way to do business among friends. The “unimpeachable” fact of the matter is that the Pretender in Chief was ultimately doing it to protect the interests of the Iranians and protect HIS negotiations with them, which was kept out of the public light for the better part of three years.
People who spent their careers in U.S. intelligence will all tell you this.