Miles3 - as you notice not every congregation has many df/da. At least the congs I've attended never had many, maybe 1 or 2 which amounts to peanuts. If they lose 35k-70k/year for DF out of 7+million that doesn't amount to much money that they would notice or make a dent in their revenue. Revenue loss may be to the problems I've stated above which don't really have much to do with df/da. I don't know of mass df, most I heard of in my congs were 2 people being df.
As had been said before, dubs are cheap and only few give more than they have to, this may have something to do with low education and lower paying jobs per capital when dealing with jws. WTS created this and they can't squeeze blood out of a stone. Mormons on the other hand have many better educated ones who generate a better revenue for their cult which wts doesn't seem to get. If wts wants more cash they need to push for higher education so the younger generation of dubs can contribue more. They can thank all wise leaders for this.