I did frequent flushes on a very tired 700R4 trans in an 86 Caprice I had when I used to work in a dealership. Because I had access to cheap fluid and a hoist I did it every 10k or so, drove it a lot of hard miles for about 3 yrs and the trans never failed, just slipped on the 1-2 shift the whole time I had it. I'm pretty sure it was the original fluid in it when I did the first change at around 120k miles.
I've done complete flushes on this and several other old cars and never had a trans fail on me yet, knock on wood.