If the ref let it continue would Joshua have gassed out again?

I agree we can only speculate. But Wlad had already been down twice in the round and 35 seconds is a long time to survive when you have been down twice and your opponent is all over you again.

 

35 seconds would have been easy for Klitschko to handle considering Joshua's gastank was empty.

Just Watch the last sequence again. From the uppercut to the break.

Joshua was tired and Klitschko already looked better than right after the uppercut.

The main difference between them are the all or nothing mentality versus the cruise control when opportunitys occur.

Blow your load when smelling blood or hold your horses on the track.