I will try to explain this simply...
Just because you can achieve insane high scores while being indestructible with the AIU doesn't mean every player will be able to do that.
Just because some players behave stupidly with her, running around ignoring the basics of the game, doesn't mean that every player will do that.
Just because you could move up to Gold with other characters doesn't mean that every player will be able to do that - you might simply be better than them.
In fact for an idea of the typical Silver players experience of stepping up to Gold without the more powerful Reckoning characters read
this post. Rather than learning 'bad habits' playing the AIU some players might at least be able to get a feel for how the game plays at Gold, learn enemy behaviour etc, without lying on their back feling like they are a burden to their team.
Maybe If Gold/Plat players feel she 'trivialises' the difficulty & it's too easy they will leave in droves. Alternatively they could just not play her. Either way the largest percentage of the player base is still on Silver anyway so the world won't end without you.
If sufficient numbers of Silver players can be encouraged to step up to Gold by characters like the AIU or Juggy that can only be a good thing. With no more DLC to come, long after you have gone, they will still be playing & keeping the game alive because of the fresh challenge of the higher difficulty they otherwise may never have attempted.