The effect of dropping the charge for the literature is very obvious from what followed in the organisation. I find it slightly odd the point even needs to be argued.
The fact that income from literature dropped considerably when the charges stopped is shown by the progressive downscaling from hardbacks to paperbacks, to fewer pages and less often, to discontinuing yearbooks, calendars and practically everything else.
The organisation scaled down its literature because the “donation arrangement” ruined their previously profitable enterprise. They didn’t scale down because contributions form the literaute was increasing under the new regime. That’s nonsensical. If contributions for the literature increased, then the incentive would have been to produce more literature, not less!
The fact that we have an internal letter that mentions this decrease in income in the early 1990s merely confirms what is already obvious by their actions: that dropping the literature charge was a financial disaster for Watchtower.
Incidentally, and almost redundant given all the other evidence, it’s also worth mentioning that in previous discussions of this topic, a number of contributors to the forum who were elders during the period have confirmed that donations following 1990 were far lower than the previous income through charges for the literature.