To be fair, I think a big part of Mercedes' dominance is that they had the best engine and the best driver, two factors that are not under Newey's control. I agree with your other points.
Verstappen only really came of age probably halfway between 2019 and today. Fast, yes, but last season was the true changing of the guard as far as I'm concerned.
Engine: yes, they had a massive advantage
Driver: Not sure, from 2019 onwards Verstappen has probably been the better driver but he didn't have the car, but we will never know this.