Following fastJehu's link on the previous page I came across this letter from the Society in 1991.
https://drive.google.com/file/d/0B4oXVPm4nrmfNnkwVUZwQXVHNGM/view
This was one year after the new "donation arrangement" had been introduced to replace charges for the literature. Notice at the end of the letter it makes the comment:
Although funds received for the worldwide work have diminished somewhat over the last year, we are confident that Jehovah will continue to bless these arrangements.
In my recollection (limited as I was young) brothers were pretty good about donating the right amount for the literature for the first few years (they kept a sheet with prices behind the desk) and this gradually diminished over time. So if the Society was already noticing a reduction in donations within the first year, then we can only assume that it got even worse in the following years.