First, no quid pro quo, much less an illegitimate one, has been proved about Trump’s dealings with Ukraine.
Second, quid pro quos have always been a feature of U.S. foreign policy — from the long-ago Louisiana Purchase to more recently President Barack Obama’s sending the Iranian regime $1.7 billion in cash, which was central to the controversial U.S.-Iran nuclear “deal.” Quid pro quos are sometimes excellent and obvious, sometimes controversial, sometimes illegal.
Third, if a quid pro quo involving Trump is established, and the agreement it refers to was the leveraging of military or civilian aid to Ukraine for assistance in the investigation of the attack on the 2016 presidential election, that would not be illegitimate. It would be of a piece with special counsel Robert S. Mueller III’s investigation, which involved least a dozen requests for assistance from foreign governments by Mueller and his staff. If the aim of a negotiation was to lever information on the operations run against the United States in 2016, it’s a legitimate ask.