To start, I'm sure majority teams have 7 dmen playing between 40-80 games a season.
Carlo played more than 20 minutes a game, all 82 games at 19 years old.
McAvoy at 19 started with Boston in the playoffs playing more than 26 minutes a game.
Colorado traded Barrie to make room for Makar so he could jump from 17+ minutes a game in last years playoffs to 20+ minutes a game in his rookie season (20 when the season started).
Hughes is playing more than 21 minutes a game as a rookie (was 20 when the season started).
Heiskanen was 19 last season. Played more than 23 minutes a game.
Werenski played over 20 minutes a game at 19.
Etc, etc, etc, etc, etc.
All these teams made sure there was room for their talented young D when they entered the NHL, and that is what all teams that successfully develop great D prospects do. Almost all top-4 D started out as top-4 D or were within a couple dozen games, or were sent back to the minors. Dermott was developing extremely rapidly on the Marlies and completely stalled out on the Leafs playing every game on the sheltered bottom pairing here (as was also the case with young D prospects in Detroit under Babcock - where the best thing for your career was to get traded away or claimed off waivers - like Quincey - after 13 games total, under 13 minutes per, over more than 3 years under Babcock). You want the Leafs to do an even more disastrous job of developing Sandin?
If this team won't develop D properly they should do them courtesy of trading them to a team that will.
And no the majority of teams do not have 7 D playing 40 or more games a season - not even close. 195 D played 40 or more games last year which means, at most, 9 teams had 7 D playing 40 or more games last season.