I think the team should be done “building”, while at the same time I think having 20 year old Mateychuk playing now is actually the best for him and the team, currently and in the long run.
You very well might be right, I really have no opinion on this since I haven't watched Mateychuk play in the AHL.
But generally speaking I'd say with D-men it's always a safer option to take it slow. It would be tough to find examples of players where you could say extra 40 games in the AHL ruined them, but pretty easy to find examples of young D-men who were brought up too soon and played minutes they couldn't handle and never became even close to the players they were supposed to become. And then people will never know if it was because they weren't developed properly or if they just were lacking in the first place.