If indeed he was good enough to play 7 minutes in the KHL, that would benefit hs development more than playing in the USHL by a country mile. The not being challenged as much pretty accurately applies to players leaving Russia to play in in the USHL(or CHL for that matter) as opposed to playing in the MHL/VHL/KHL. There are quite apparent recent examples of that and how it works out.
My beef with German Rubtsov leaving was that he was running away from tougher competition in Russia(if you want to compare leagues, then here you go: he's had 15pts in 15 games in the MHL, then 22pts in 16gp in the QMJHL in the exactly same season after leaving. That culture shock and trouble adjusting to the tough pro level game in NA must have boosted his performance
). He was a promising prospect in the system of one of the weaker KHL teams. He was close to getting his chances in the KHL(in fact he does have 15 KHL games as a 18y.o. on his resume). That would have been exactly the point in his career where he would have to give it some effort to deserve minutes in the second best pro league in the world. Instead he decided to spend another one and a half seasons in cosy juniors in NA and then until now was playing in the inferior to the KHL AHL not showing any signs of progress.
The "pro style game on NA ice" is exactly what players would get playing in Russia being able to advance to the VHL and KHL much earlier if they bring it and many KHL teams changing rink size(which I am not a fan of personally). It would have been defnitely better for Guschin to not be stuck in a junior system that is not better than the MHL by any measure. Playing time seems to be your only valid argument, but that again implies that there is in fact LESS competition in the USHL as the saame player would have it easier to get ice time and assignments there.