Did you know that the only reason automatic transmissions feel smooth, is that they are programmed to be in two gears at the same time during their upshifting and down shifting.
During this transition, the clutches are slipping and fighting each other. wearing down the clutch material.
If you want your Automatic to last 300K while towing a 3000lb trailer, just put a "shift Kit" into it which eliminates the transition, and makes the trany shift like a manual trany would.
Just out of curiosity, when did down shifting for engine braking become a bad thing?
Then again you might be the type who would drag their brakes while decending Pikes Peak.