It depends on what you mean by being brought down. I can see splinter groups meeting in private homes discussing and believing WT doctrine for decades after the WT organization collapses (if it does). If you mean, ceasing to exist in the form and size they are now, nothing short of dropping all their unique teachings ala WCOG (which by the way is STILL around under a different name) would do it to the extent that they disappear. I doubt that would do it either. Will they get smaller as the years go by? Yes, I think the evidence points to them hitting their high water mark. But, I just don't see them disappearing. Look at the number of fringe religious groups out there. Now with the internet, you can hold virtual meetings that connect small groups from across the globe.
Tithing - I agree with Morph. It would be a shock at first and fringe folks like me could use it as a reason to leave. But they don't care about those people anyway. If you can convince parents to cut off their own kids, you can certainly convince people to give you money.
Shunning - dropping that would definitely impact the numbers as faders/fringe JWs could walk away without repercussions although many hard core JWs would still shun them. It's simply too ingrained in the culture. It would take an entire generation or two for the shunning to stop.
Doctrine - LOL - They make changes all the time with little to no impact. Most JWs don't even know their latest teachings.
