So we're going to no-true-scotsman our way out of this?
Nothing is binary; I'm not going to say "either leveling helps a game all the time or it doesn't". Sometimes leveling-up mechanics help a game, sometimes it doesn't, sometimes it alienates one set of users and attracts another, and there's no magic formula for when it helps and when it doesn't, and the nature of the leveling-up mechanic itself and how fun that is can vary in effectiveness dramatically. It's like having the user's avatar wear a funny hat.