Ok, so perhaps I wasn't clear on some things. Let me explain the saturation I mentioned and the primary cause of transmission wear. In the transmission are bands and clutches that engage each other to transfer motion from one part of the transmission to another. The clutches in an automatic transmission work just like the clutches in a manual transmission. As the friction surfaces make contact, there is always a moment where the speeds of the two surfaces are different, which results in some slippage until the surfaces speeds are synchronized. The fluid to some extent lubricates the surfaces as this occurs, reducing wear, but some friction material always breaks down and wears off. This friction material has to be carried away by the fluid, otherwise it would simply build up and prevent solid engagement. The fluid is designed to hold a certain amount of this material in suspension preventing it from collecting somewhere else in the transmission where it could cause damage. But the fluid can only hold so much of this material in suspension before it becomes saturated, just as there is a limit to how much salt you can have dissolved in water at any given time.
This friction material in the fluid is made of very small particles, most of it will pass right through a transmission filter. As a result, the only way to remove this build up of material if to replace the old fluid. This saturation point is reached in roughly 30,000 miles under most conditions, regardless of whether the fluid is a synthetic or not. Synthetic fluid can withstand greater heat (the trans should not be exposed to this anyway, the seals don't like it) and has better lubriscosity, which is great for the bearings and other moving parts, unless the oil is saturated with friction material, in which case it becomes more like a polishing compound.
So the limiting factor in a transmission is heat and the buildup of friction material. Both conventional and synthetic fluids are excellent at protecting the moving parts, both are highly engineered to have specific amounts of certain friction modifiers, detergents, and other stuff they put in there and the oils themselves will outlast the 30,000 mile interval if it were not for the buildup of material.
So not to argue, but many can get 200,000 mile or more out of the original trans regardless of the fluid if it is replaced at the proper intervals. I think at that point, the synthetic transmission fluid is more of a feelgood thing than any actual results.
I do prefer synthetic in my engine, I only run group IV stuff in my engines. I also run extended oil change intervals. Why do I feel differently amount this in my crankcase? There are no friction surfaces in the engine. The requirements are very different. With the proper oil, the right filters, and the proper maintenance schedule, you can run well beyond 10,000 miles between oil changes in an engine. Even so, you will not leave the fluid in the engine nearly as long as you would trans fluid.
So part of my difference in feelings? I could run conventional oil in my engine for 5,000 miles, or I can run a good top quality full synthetic for 20,000 miles. Since the fluid does not cost 4 times as much, it saves me money, while still protecting my engine better. But with the transmission, the fluid change intervals are about the same either way, but one costs a lot more will little benefit to show.