Comparing a manual transmission with an automatic transmissions reminds me of the old saying about “you can’t add apples and oranges”. Rather than comparing the longevity of a manual transmission (almost unlimited, IMHO) with an automatic transmission, you should be comparing the clutch on a manual transmission with an automatic transmission.
I have owned two cars with manual transmissions (a VW Karmann Ghia that never needed attention to either the clutch or the transmission in 95,000 miles and a Chevy Citation that needed transmission repair twice and clutch replacement once in 60,000 miles–an apparent testimony to GM engineering in the early '80s!)
On the other hand, I have owned 6 cars with automatic transmissions, and none of them ever needed to have the transmission replaced or overhauled, even up to 150k+, with the one possible exception being a lousy Volvo that needed to have the seals replaced several times due to chronic leakage.
So, in my experience, automatic transmission cars are no more prone to repair in a given number of miles than a manual transmission car is. However, a lot has to do with the specific brand of car (Chrysler products made over the last 10 years or so have lousy automatic transmissions) and the maintenance that the car is given. If someone doesn’t perform the specified maintenance on his automatic transmission, then he probably deserves whatever failure might result.
If you have a car with a decent record for transmission reliability (you didn’t tell us about the car in question!!), and if you maintain it properly, you should be able to drive it for well over 150k with no problems in regard to the automatic transmission. We have come very far from the days of the Buick Dynaflow!!