Just to jump into this, many people often associate development of all players as though all players played goalie. For a goalie, I think the consensus is that they need to play lots and face many shots to develop their overall game. So for players, it makes sense that they develop their (offensive) game by playing lots and taking many shots. It is a line of logic that I don't disagree with.
I don't think you can be so quick to dismiss it by simply saying there is no evidence for it. There are not parallel universes where we can see a player follow both paths. But there are plenty of players who on consensus have succeeded by marinating in the minors, or busted by being rushed. Sure it can be player specific, but there are also rules of thumb.
There is neither evidence for or against. The point is that we can't assume that he would automatically be better had he played longer in other leagues, especially as a direct response to some fans' feeling that he is not meeting expectations. There is just no way to know, or even test it, and no reason to assume.
Wait now we have to be factual... I missed that in the rules. Apparently I did not read the rules. I think both sides to this argument are opinions. At the very least I am sure you might even share the opinion that some players (across teams and years) have been rushed into the NHL. That opinion is influenced by a pattern of players excelling prior to jumping from junior to the NHL and perhaps coincidentally not living up to expectations in the NHL. The player loses confidence and cannot /does not develop skills that people expect they should. An opinion that takes the side that they did no harm to their game by coming into the NHL at a young age when they could have stayed back even though there are clearly elements of their game that need development is no more factual.
The point is that Lazar is not meeting the expectations of some fans and therefore he should have played longer in juniors and the AHL because then he would have met their expectations. There is no basis in fact for a fan to hold onto this as true. Sure it may have had a positive effect, but just as likely to not have. All we're left supposition.
People assume that longer development in lower leagues means a better NHL player, however, this is not actually a general rule. Some players need more time, some don't. Some are ready to make the NHL, like in Detroit, but can't because there isn't room.
If you listen to our staff, time and time again the thing holding a young player back from NHL play time is not offensive honing, it's the ability to play an all around NHL game, and how not to be a liability on the ice while you learn the NHL game. It's why Puemple was sent down, Dorian said that his skills were good but he wasn't consistent enough to be in the NHL yet.
My opinion in the end is based on Lazar's play. I think he's been doing a good job for a 20 year old player. The only part of his game that is lacking really is his offence, and he isn't going to become a better NHL scorer playing in the AHL.