That isn't average. I've never had an automatic tranny in anything I've owned fail with less than 120,000 miles on it but I'm good about getting them flushed every 30k too.
My sister has a '99 Explorer which receives little in the way of any maintenance other than oil changes, her and her husband tow a enclosed trailer with a Harley in it all over creation plus she drives a lot for work, and the tranny didn't crap out until about 95k and I guarantee it had the original ATF in it.
OTOH, my secretary's husband bought a brand new Ranger two years ago, 4-banger, automatic, and had to get a new tranny about a month after he got it. :blink:
One of those things that you just never know.