I think that it was a hard fight to score and I'm not surprised that the judges all had different scores. It would've been very risky to go to points for either fighter.
Usyk grew into it and definitely started to take over, but the turning point was probably the punch at the death in the 6th.
Bellew got sloppy and it rattled him. He wasn't the same after that and a series of body shots really seemed to take it out of him.
Bellew even said himself he thought he was behind so started to go for it in the 7th. How can you be up by 3 rounds and think you are behind after 6 rounds? Maybe he was concussed from really early in the fight.
As you say if it did end up going on for longer Usyk would've just kept winning rounds so would probably have won on points anyway.

