honest question but why did Koski stay so long in the KHL. Your other examples didn't stay in Europe long at all. They got snagged by NHL clubs quickly. What was missing in Koski's game that no one seemed to care to sign him for so many years in the KHL?
He is far bigger goaltender than Raanta or Niemi or any regular sized goaltender. Hence it takes time for him to get comfortable in his body. Similar phenomenom can be seen in football's huge target-men who tend to breakout at older age even if they were playing in semi-professional or amateur circles the time before.
Koskinen then was earning NHL money and had a huge role in a free for all contract system where the highest club in the pecking order chose to rely on him for years. Longevity in the KHL is not exactly guaranteed while in the NHL thanks to guaranteed and restricted contracts goalies can hang in the system far longer.
In NHL there is still some strange belief that only someone who is located in that continent can only be judged. Despite that the huge anticipation for talent of Rask, Lehtonen, Saros, Thomas and Kiprusoff was already witnessed years before already in the SM-liiga. Koskinen wasn't in this category of ethereal goaltenders when he came over but so aren't about every current number one goaltender in the Northern American league either. There is no chance that Koskinen wasn't among the best 30 goaltenders in the world during 2013-2019.
Yes, NHL is better league than the KHL. But to be a better team has nothing to do with being in the NHL when it comes to coaching and environment. You of all people are experts in that.