The only think that will stop a large-enough religion like the WTS is financial issues and even then there are enough zealots who would sacrifice their own financial well-being to bail them out so it would have to be sustained issues to cripple them.
And at some point they would simply walk away from the corporate structures and setup new ones, have a variation on the name and be right back in business.
Even if the GB announced they were wrong about everything and had simply been making everything up - people would still believe it and follow them.
The best we can hope for is really a slow decline or stagnant growth and aging membership so it will die a natural death. It's unlikely though. Once a religion gets to a certain size it's almost impossible for them to fail - just look at Scientology.