...Transmission fluid doesn't need to be changed or flushed very often at all. What you saw was a way for the
dealer to get easy money, offering a "warranty" if the customer spends a bunch of money on unnecessary tranny flushes. Clever but unethical. A huge waste. Transmission fluid normally lasts for 100,000 miles or more.
If viscosity factors into proper operation of an automatic/manual transmission, then going to mileage extremes could be harmful. The additive package disappearing or becoming weak is another issue. On my DD the Merc V trans fluid viscosity probably shears to min spec (or lower) by the 30K recommended OCI. Let's say it sheared from 7.4 cSt (100 deg C) down to 6.0-6.2 cSt in 30K miles. After 90K miles on the same fluid I'd expect approx 4.0 cSt.
ATF shear rates
There might be some fully synthetic automatic trans fluids that can get you 100K miles. But with semi-syns or conventionals, that might not work.
I 150% agree with you. Most OEM ATFs are a group 3 and just get "bet the heck" by heat. Even Amsoil group 4 Signature Series can get hammered by heat and CAN start to varnish because it mainly is a PAO and PAO's varnishes on high heat. This is where Redline's base stocks that are a group 5 and are far better at taking high heat with far less chance of varnishing.
Example: Our 2014 lease return Rav4 from a lady in upstate New York with no sign of soccer mom hauling. I drained the horrible OEM Toyota WS [censored] out and it was dark dark dark maroon/blackish at 21,400 miles. The smell was SO BAD my garage smelled for days. I even got sick will changing it and had to open up the 1,000 sf garage to get air it was so bad. That OEM ATF was toast at 21,000, no hitch and no sign of any kind of hauling. The whole back of the Rav4 was like it came off the showroom floor.
That's not my experience at all. I changed the Dex 6 at 50,000 miles-25,000 miles of which was towing my 5,500 pound travel trailer around the country. Even at 50K-it still looked nice and red. And this was with high tranny temps under some towing conditions.