That's not entirely true. Whenevr a player sticks with Montreal and doesn't go to their AHL affiliate they usually have a lot of success. It's the players that go down that can't do anything and don't developp properly.
In recent years:
Victor Mete - no time spent in AHL, trending very well already a top-4 D
Noah Juulsen - only half a season in AHL, sticking in Montreal, looks to be a potential top-4 D
Brendan Gallagher - only half a season in AHL, 1st line NHLer
Artturi Lehkonen - no time spent in AHL, top-6 NHLer
Alex Galchenyuk - spent no time in AHL, legit top-6 NHL talent
FAIL:
Jacob De La Rose - spent parts of 3 seasons in AHL, could never developp an offensive game
Charles Hudon - 3 seasons in AHL, still cannot cement a spot on Habs' top 3 lines
Nikita Scherbak - 3 seasons in AHL, still cannot cement a spot in Montreal
Brett Lernout - 3 seasons in AHL, unclaimed on waivers last week
Daniel Carr - 4 seasons in AHL, management let him go UFA
Michael McCarron - 3 seasons in AHL, still can't stick in NHL, unclaimed on waivers
Zach Fucale - 3 seasons in AHL, has never developped
Kotkaniemi seems like he's going to belong to the first group. Anytime they decide to ''rush'' prospects it turns out great. They have good judgement for that stuff.
Also there is 2 first rounders and 2 high second rounders in the ''fail'' category, whereas 3 out of 5 in the ''good'' category were not even 1st rounders.