The fluid and filter on my '88 Grand Caravan has been changed once in 217,000 miles and I use it regularly to pull an enclosed trailer that's bigger than the van! That's not neglect, that's abuse, but it shows what the old hydraulically-controlled transmissions were capable of. The newer computer-controlled transmissions would be a neat idea except every manufacturer has found every possible way to make them cheaper instead of more reliable. Flushing the fluid is a good idea to get any varnish or other contaminants out. The fluid contains additives that last a lot longer than those in engine oil, but when they do become depleted it has a bigger effect on these transmissions that need all the help they can get.
In the past we were happy to just drop the transmission pan to replace the filter and the half of the fluid that came out. That was recommended every 36,000 miles. Ford was famous for extending that interval to 50,000 or 60,000 miles to make it look like their cost of maintenance was lower than everyone else's. In the fine print though, that was for light duty use. Under severe duty, their schedules were the same as those of the other manufacturers. Severe duty includes driving in town, on dusty roads, and extended highway use. Go figure. Today a flush, which replaces 100 percent of the old fluid can be justified at least every 50,000 miles, or more often if that's what's recommended in the owner's manual.