So your premise is that the Blazers front office that trading Dame for Jrue would make the Bucks worse? And that they believed this because Giannis tricked them that he might leave? Wouldn't Dame going there pretty much assure that Giannis would stay?
Sit this one out.
If you want to go a little older, Len Bias is probably the biggest what-if. He is pretty much a can't miss prospect going to a good Celtics team that still had Larry Bird. He might bridge the gap over two generations of Celtic players.
More recently, if David Lee doesn't get hurt does Draymond ever become a significant contributor to the Warriors? People might forget Lee was legit good. He was an all-star and third team all-nba player. There's a reason Draymond wasn't getting playing time. Lee hurts his hamstring and Draymond enters the starting lineup never to leave. But Lee's not losing that starting job if not for the injury.
Here's a thought experiment. With his present athleticism at what height would Jokic not be able to dunk anymore? If he was 6'6 could he still dunk? 6'4?