External Transmission cooler?

[JCL]

You already have a thermostatically controlled heat exchanger for the transmission fluid. Standard from the factory. It has the added benefit of heating the transmission fluid on cold starts, to reduce wear. Overcooling would negate that benefit.

Your failure wasn't necessarily due to a lack of maintenance. There have been more transmission failures than we would like to see, statistically, but there is not a common cause or theme for them. Changing fluid has not been shown to improve transmission life. You saw with your transmission that while the fluid was burnt, the clutch plates weren't. That suggests that the clutches weren't slipping for any length of time, rather it is more likely that something broke and it overheated suddenly. Look to that bearing.

I think one data point would be to consider posts from withidl, who towed 8300 lbs with his 2000 4.4, over the continental divide, for tens of thousands of miles. No transmission issues. Lots of posts here to read about. Due to that and similar examples, I really don't think heat is the problem with the transmissions, as long as the cooling system components are working as designed. Rather, I think that the failure of individual items (actuators, seals, snap rings) leads to premature failure prior to wear out. I think that the complexity of the transmission is the single biggest issue with it.