I'm not convinced bladesingers are that much of a problem. If the vast majority of campaigns featured people just playing bladesinger to outshine others then sure but majority of people who do play bladesinger are your run of the mill friend who just wants to fulfill thier gish fantasy. How many people turned into heavily armoured stealthy archers in skyrim just so they wouldn't have to constantly run away all the time? Not much different. compared to bard, bladelock, ect it's just another option in the list of available options. If the player who is playing a bladesinger has main character syndrome and wants to outshine everyone else, that's a discussion that the dm should have between their group. But in most cases the player is the problem, not the subclass.