My two cents (no pun intended):
...If that was the case then that $20 billion could all be used up in less than 80 years...
That is only true if :
(a) they will stop having revenue for 80 years
(b) they will continue the printing business as it is now
(c) they will continue being a religious nonprofit organization
Right now things don't point in any of those three directions. They have a great established real estate business that they started with minimum investment since all the properties were given to them absolutely free (almost $0 investment for billions of dollars in real estate). That business isn't going away any time soon. That is, not to mention their current prime real estate that they currently have that in any event, they can sell and make more money.
All the re-branding and changes they are making may look like an event of epic proportions to active, indoctrinated JWs, but what they are doing is nothing other than what every other single company in the publishing business is doing, that is, repurposing their resources, changing directions, and pursuing ventures that are more lucrative.
Furthermore, non-profit organizations can own business. That's nothing new or unheard of.