You said a "few big performances on big ice". It is a lot more than a "few big performances". It is "2 seasons worth of big performances on big ice". There are also a lot of guys who move from Europe to the NHL and have success; even guys who seem like they are only made for big ice.
Besides, IIHF is on NHL-size rinks this year.
At the congress in Rome, the International Ice Hockey Federation (IIHF) has voted to play the World Championships on NHL-size rinks as of 2022.
swisshockeynews.ch
As for your last statements, that is matter of opinion and somewhat factually untrue as well. Dubas was the guy who brought in Marchment in the first place. He has brought in various guys who have that crash-and-bang style, even if they are not large, and has even brought in guys who were transformed from pure playmakers into those types. Dubas just wants guys who can skate and bring more value than crash-and-bang, because believe it or not, guys who just crash-and-bang don't have much of a place in the NHL anymore. You have a few leftovers who are usually in their 30's now (i.e. Simmonds, Reaves, and Maroon), but if you can't skate, score, provide defensive value, etc. then most teams don't really care if you are a big, mean guy. Marchment struggled to bring many of those things, and his skating was especially concerning. Korshkov actually brought some defensive value to the table, and had better raw offensive ability. One was able to find a way to work out. The other (so far) has not. 9/10, a guy like Marchment turns into another Korshkov. Dubas is not losing sleep or worrying about the odd guy who can turn into a late bloomer with another team, especially when he gets a guy who has a profile that has turned out to be successful in the NHL (and especially here) far more often.