The whole "we woulda won if so and so NHLer was there" would make about as much sense as a Kentucky or Duke basketball fan saying "we would have won the NCAA Tournament if all those 1 and Dones had hung around through their senior year"
Once you're in the NHL, you're not a Junior player anymore, imo. And at the end of the day, winning a tournament just means you won a tournament. Czechia isn't the number 1 hockey country because they won the world championships last year, it just means their team beat all the other teams that were there. You can look at trends and see how much better USA is doing at the U20 level the last 20 or so years compared to the prior 20 and draw what you will from it. Winning one individual tournament doesn't necessarily mean they're "better" just that they have bragging rights.