You think the progression in Jeff Petry's game is a normal development curve? When we acquired that guy he was a bottom pairing defenceman or a good 7th in his mid 20's. He obviously developed into one of the better defenceman in the league and a top pairing guy. That is the definition of exception not the rule.
If you expect some similar explosive development out of Harris' game than you can say the exact same thing about Struble, Xhekaj, Fairbrother, etc or almost any other young defenceman for any other organization, anything is possible just exceedingly unlikely.
To say any prospect will be better at 25 than they are at 21 is completely obvious, you aren't telling anyone anything. To expect a Petry like development arc is something else entirely, you can't go through the league and find 5 guys that have that kind of late 20's development at D, it is very rare. I will just bet on the side of probability and say Harris isn't gonna ever do that. If he hits that less than 1% outcome probability I will be over the moon, but he won't. He is hopefully a stout bottom pairing D in his prime, we shall see.