Agree with this mostly. Only thing is that I don't think he was rushed originally, as the Habs had a spot and he was able to play in a responsible way and not just survive. He was learning the defensive aspects of the game he would need in the future and doing it in the toughest possible environment. I think it only made his progression faster on that area. And he also saw what he needs to improve to be able to become a top6 C. The end of the season was a bit odd, "fatiqued" and into the stands. If they thought he was too tored to play in the NHL, I think he would've still beem able to play in the A. But whatever.
Next season was a disaster, he tried to bulk up all he could and lost his legs, so there was nothing to work with. He was just trying to survive, and failing a lot too. He should've been put in the A quite fast to get minutes and get his feet up to speed. And the groin injury and concussion came into play too. Then he had a good summer and strong play offs in the bubble.
Another off season (kind of) and more training. Then to play in Liiga, which helped him to get a good start for his third season (even though his time here in Liiga was a bit disappointing, but it was his training camp sort of). So this third season was somewhat what it should've been in terms of playing. Decent production at first too. But the line lottery he was put in at some point didn't actually help him, as it sure didn't help the wingers that were in it. Him being put of the lineup in the finals was a bit strange too, well, not strange if your coach goes with the old time hockey gamebook and plays vets over younger ones no matter what their results are.
So to sum it up, I think all he could have gained for staying in Liiga for his first season might have been the gained strenth. So in my opinion the things he learned that season in the NHL outweighs that by a mile. His first off season training was a bit questionable, too much focus in bulking up and gaining strength (or at least gaining it to "wrong" parts), should've maybe shift some of the focus in his skating technique (and endurance at the same time, as Barkov said that when he started working with the figure skating coach he thought he would die at first).
That and his second season is something I see that could've been handled better. Last season he maybe should've got some chances to play in Danault's spot as he was not actually lighting it up. And KK definitely isn't a liability in his own end, if not at Danault's level yet. Keep the puck at the other end and you won't have to defend...
But as your main point originally was; KK was drafted as a long term project, everyone was talking about 5 years minimum. He's on that path.
And to some posters; why didn't they take Tkatchuk instead when he was the more ready player? Because they saw that KK will be the better player in the long run and in a more valuable spot. We'll see if it happens or bot, but I have confidence in it as they adapted his training for his second off season etc.