I don't particularly disagree with you, however there is definitely something to be said about changing the scenery negatively affecting a player's development. William Eklund looked fantastic last year in his 9 NHL games, and suddenly struggled mightily once he was demoted overseas. Rarely is there a 1-to-1 correlation between top performers in the WJC and NHL-readiness (case in point: Mason McTavish).
Marty St. Louis said that once he thinks the NHL is too much for Slafkovsky, he would consider the WJC. Once he sees him no longer implementing the little things, once he struggles with the pace, etc. However, until now, there hasn't been much proof of that except for his issue of keeping his head up, which everyone will admit is certainly a concern, but may be an ongoing development that could be undone by going to play inferior hockey. At that point, it's up to the NHL staff to evaluate if the WJC will help/hurt his development. Some would argue that the increased confidence is not worth the undoing of some crucial teaching moments in the highest-caliber hockey of the NHL.