Saying "no" to cash isn't how an organization gets to have $100 billion.
Yeah - my post was tounge in cheek.
You generally amass 100B by spending less than you bring in and investing the excess well. So, if you think that's a problem, either encourage the organization to bring in less or spend more. In this case, spending more makes more sense to me - as long as the increased spending is on worthy causes.
My best guess is that Watchtower's 7B per year number for tithing receipts is probably in the ballpark (looking at just the US: 7M members, call half of them active, and half of them wage earners making the median individual income, so 1.75M people earning 30K per year, making 5.25B in tithing - maybe a few billion more from Europe. Most of the rest of the world is probably a rounding error).
My guess is that they are spending in the ballpark of 5B - probably less than that - (charity, operations, property management, new construction, education, etc). Don't really have anything to back that up.
They currently give roughly 1B per year to charitable causes. That could easily be doubled, tripled, or more, and I think it should be. They apparently are very careful about how to go about giving the money so that it isn't wasted, but it shouldn't be too hard to figure out how to give more. The Gates Foundation has figured out a way to give away 5B per year, so the church should be able to do the same.
Eliminating tithing is a non-starter though, for reasons already given.