As long as there are people buying into their paradise dream, they'll continue to go on and prosper. They're not some silly small movement. They're big and they'll get bigger, just like any other mainstream religion.
That's how I feel about it. There will always be some percentage of the earth's population that doesn't want to think for themselves, wants to believe that soon all of their problems will be solved and all they need to do is listen to the "man behind the curtain." On the other hand, because of the internet and ability to easily access site's like this one, the Watchtower will no longer appeal to middle and upper class. This probably worries the Watchtower because these are the people with the cash.
I don't know about them getting bigger. It's really going to come down to what changes they make. If they lighten up about the blood issue, come clean 1914, 1925, etc, encourage community involvement instead of just field service, - then their numbers will grow. If they revert back to the hardline that I recall (such as no college, etc) their numbers will fall. My prediction is that their numbers in the US and Western Europe will slowing drop, while they'll have gains in other parts of the world. I think China will open up more and this will be ripe territory for them.