In my 40 years of driving I have never had a transmission fail, manual or automatic. I typically keep vehicles 10 years or more and put over 200,000 miles on them.
This is not to say they have been totally trouble free. The manuals have had to have clutches replaced in the 150,000+ mile range and I did have a pressure plate release finger break on my Barracuda after approx. 20,000 miles. The autos have had have modulators changed and sensors replaced but nothing that left me stranded or required the tranny to be pulled.
I currently have (4) vehicles three with automatics, one with a 4spd manual. If I could only have one an automatic is much easier to live with.