But shunning is literally the only tool they have to really force people to comply. If not for that, how would they enforce their crazy rules? That would be the single largest fundamental change they've had in a very long time. Sure, it would be great if they dropped organizationally mandated shunning although it would take at least a generation to change the behavior.
To drop a scripture, Jeremiah 13:23 - can a leopard change its spots? Or can a cult not be a cult? (or high control religion if you prefer more politically correct language).
If they morphed to a mainstream religion, they likely could grow it but only after suffering major losses and totally rebuilding. Not saying it can't happen but wouldn't wager anything on it.