I think the problem began decades ago, when the steady downtrend of partakers slowed to a near-stop. This is when they opened the door just a bit, by claiming that there could still be partakers after the 'doors were closed' in 1935, to account for replacements. Once that door was opened, more people 'heard the heavenly calling.' The numbers began to stay flat and occasionally increase.
The WTS then opened that door just a bit wider, by claiming that they couldn't say anyone was wrong to partake. They understood that this stance was risky, so they basically said that new partakers might be mentally ill. But that didn't do anything to slow the increase in partakers, to the point where it was no longer salvageable. Once the number had increased from ~9,000 to more than 20,000, it was clear that the doors had been thrown wide open.
The problem is that this is one of their core teachings, and fiddling with it is risky. New explanations will replace and supercede older ones, in which case the rank-and-file might have some questions. Such as wondering about the old 'replacement' explanation. Or the old 'mental illness' explanation. Presenting it as new light from Jehovah just shifts the blame-- now it's god's fault that they called people crazy for partaking? Oof.