Except that Burisma was already being investigated before the call (they re-opened certain investigations). So Trump was asking Ukraine to investigate corruption they were already investigating? He needed to withhold aid specifically to target the Bidens. Keep digging that hole my friend.
If the investigation is already in progress and Trump -- according to you -- withheld aid to ensure that the investigation continued, how does that qualify as quid pro quo?
Ukraine is known for its corruption. It might pass as a quid pro quo if Trump withheld aid to stop the investigation -- which is what Biden is guilty of doing -- but how could it be quid pro quo if he threatened to withhold aid if their corrupt government stopped the investigation?