I have a mechanic friend who is retiring this year after 30+ years. He was servicing the transmission of my Ford last week (PM per the manual). Just dropping the pan and replacing the filter and 4 quarts or so that came out.
I asked him about not changing the fluid on a high-mileage transmission that has never had it changed. He said, "That is the rule of thumb."
I asked him if he ever personally saw a transmission fail from that? He said, "No. Well, one, but that transmission already had problems."
He said he used to work at a Firestone and they did flushes every day. Except for that one car they never saw any issues.
So basically we came to agreement that if your transmission is running fine a flush won't hurt it. If your transmission is having problems a flush could help or it could kill it. Maybe start with dropping the pan and taking a look at the magnet. If it is covered in metal shavings (more than normal) probably best just to change the 4 quarts or so and ride it out and start saving for a rebuild/replace. If it looks ok then maybe try a flush later.
Just watched Wheeler Dealers where Ed serviced the transmission of a Porsche Cayman S with high miles because it wouldn't shift and it solved its problems. Just by dropping the pan and replacing 4 quarts and the filter.
I bought a used WK with 87k miles. Flushed it at a dealer with 95k miles. No problems whatsoever now at 104k.
Honestly, for the price, I'd just do a flush and get all new fluid. I have no idea what those filters filter. They don't look like they do much of anything.
From my limited experience this is where the problems arise. When people extend their fluid change interval to the max and then have the "transmission flush" performed, I've seen some transmissions fail not long after as this "flush" usually breaks loose any particles/contaminants/debris build up and then gets pushed through the transmission clutch packs or lines and clogs the lines causing the transmission to fail. While it does not always happened, I've seen this before. Just my $.02