See, though, that argument cuts both ways. What happened to DeAndre Yedlin when he left to go to Europe? Or Christian Pulisic? It seems to be working out alright for them...
The thing is, a player who takes the step up and struggles will always be accused of lack of ability to see that they should have stayed put, but a player who never even tries to play in the bigtime will spend the rest of their career being accused of a lack of ambition, so it's not like by staying here Jack will avoid any questions over his decision-making and his personality. On the other hand, on top of this, if he stays put he will have to grow old telling his grandkids about how he might have had the ability to turn into one of the best wingers in European football but never found out because he was too scared of finding out he wasn't. There are multiple other factors I could mention, like whether he dreams of representing his country but might be held back from selection if England managers don't believe MLS is competitive enough, and so on. Point is, it's not the straight forward "wood through the trees" decision you make out.