Scientology has rich members and are experts at extracting their money.
For example, in response to an unprecedented appeal for money by Stephen Lett in 2015, where he said WT had an urgent need for extra funds, JWs on average responded by contributing less than $4 extra each. That’s probably less than most people carry in loose change. (Those who still carry change. I do.)
None of what you said doesn't mean that Watchtower won't continue to exist, even if they run out of money. They might shrink in numbers, have to sell off properties and downsize, but that doesn't mean they're going to "collapse" into oblivion.
And keep in mind, Scientology doesn't have nearly the amount of followers Watchtower does. They're estimated to have some 50,000 members. Let's say all 50,000 of those Scientologists are what you are referring to as "rich members." Watchtower would only need 0.625% of their 8,000,000 members to match Scientologists.