To be precise: Trump achieved the end-goal he was seeking, which was smearing Hunter and by implication Joe Biden. He did not achieve opening a corruption investigation on them, but there are two ways to skin a cat.
I've seen so many memes on this very site calling Hunter Biden a crack-head, etc. and it is all because of this. He just never would have made the news sitting on a company board in Ukraine except for that phone call
I'm positive Trump's Administration can and has withheld or threatened to withhold foreign aid in other contexts, as have Obama and many presidents since.
But when you're doing it to smear a political adversary? That's an abuse of power
You are the one who would be screaming if Obama called up, say, the President of France and asked him to "look into" Trump's children the year before an election, on the basis of sheer rumor, and withheld billions in foreign aid to France while doing it