I always thought it was weird that we were able to kill Nozdurmo in End Time. This dude has literally seen all of time. Therefore i refuse to believe he could have been killed if he didn't want to. But the question has always been why?? After watching his interaction in legacies i think i am starting to understand.
He did it when Chromie was at the threshold of becoming an adult. Until then Chromie blindly followed Nozdurmo and only went through time to observe and preserve the flow. Afterwards Chromie starts taking on all of his responsibilities including battling N'zoth with the other aspects. Nozdurmo groomed Chromie to be his replacement.
But once again the question is why when he can see all of time. It is partially answered in End Time in that this was the best future he could forsee. Now he was corrupted by Old God magic. So naturally we all think that his best vision of the future is to ally with the old gods so at least a corrupted version of life exists.
however going back to when he chose to die. He did it when Chromie was coming of age. He releases the Dracthry when dragons finally come a mutual cohabitation with the sentient races. Having them all grow up and learn to work together.
I believe at some point Nozdurmo is going to take the hit for azeroth and needs us to be strong enough to think for ourselves without him. To fight back and use the power of friendship.