NOTE: I have anecdotal vs. empirical data on this, so please understand that caveat.

I believe the problem which is actually being discussed is doing a transmission flush when it has never been done before (or with long periods without). The theory is, over time, buildup occurs within the transmission when flushes do not occur at regular maintenance intervals. If you remove this buildup, you leave gaps in the soft parts (clutch material) which means less material for the transmission to work with. This causes faster wear in the material which is left, which causes the transmission to need rebuilt sooner.

If the transmission has had regular flushes, this buildup does not occur as well as wear/tear not occurring due to buildup.

Please note, changing just the fluid (dropping the tranny pan method) under the same conditions (long period than specified between fluid change) would not have this same effect. This is because the buildup solids would still be in place. The reason a flush could be more detrimental is because (to my understanding) the way a flush works is by forcing fluid through the system backwards, freeing up any solids which may be in the filter or elsewhere and forcing them back out through the system. This not only cleans the filter, but also replaces all fluid in the transmission. When you drain by dropping the pan alone, you are only changing out the fluid which resides in the pan. There is still a large quantity which remains in the torque converter.

To this end, I've seen transmission which get flushed, having never had the fluid changed, destroy itself within a few thousand miles. Yet, the same type of vehicle, if left alone, would have lasted many, many more miles without the flush. Mind you, as stated, this is subjective. Bottom line here is that rule of thumb dictates, if you haven't kept up on your transmission maintenance, don't do a tranny flush. It will more than likely destroy your transmission. Getting scheduled maintenance on an automatic transmission is by far your best recourse.

There is a lot of debate about late ATF changes killing transmissions. To find the real answer you would need a statistically significant number of vehicles for each transmission, and a same-sized control group, driven identically for hundreds of thousands of miles... then one group serviced, the other not, then all driven identically for another hundred thousand miles... that's not going to happen.

The best I can offer you is some advice.

For older transmissions with deferred (heh) or without any scheduled maintenance, I take the Honda approach. Remove, measure and replace a small amount, about a quart/liter. Drive for several thousand miles and monitor performance and behavior. Then replace a larger amount, maybe 2 quarts/liters. Repeat driving and monitoring.

If a filter exists, replace it only after several cycles.

The approach I started on my then not-so-old vehicles is simply replace 2 to 3 quarts of ATF every oil change. Now, they are getting old, as in 150K miles each, and both have perfect transmissions. I just keep doing the same thing, also replacing transmission filters occasionally.

Transmission wear can be tracked primary through vendor-specific electronics and ODB2 codes, but in OBD2 anything in the Transmission section with the terms Range/Performance can be useful.