When it comes to development and rookies making the team, we can't make definite general assumptions based on the past. Each situation and individual has to be evaluated properly.
We won a Stanley Cap in 1986 with 6 rookies, who significantly contributed. Had we made a general sweeping statement like "we can't have too many rookies on the team", we would've never won the cup.
I don't care what happened to KK or Galchenyuk or anyone else. What happened, happened to them. Slaf, Guhle, Harris and Xhekaj should be evaluated on their own, and if there's stagnation or regression, then send them back to Laval. If they show that they belong and are progressing the right way, then they stay.
Decisions should be based on a constant evaluation of their play, not the past or what happened elsewhere.