Originally Posted by DougEFresh2

So for starters, this isn't a "this is what you should do" guide, it's a "this is what I do" guide. The 2016 Toyota 4Runner Warranty and Maintenance Guide says to "Replace automatic transmission fluid" every 60,000 miles under "Special Operating Conditions". I have never seen any other official interval of changing the ATF from Toyota for a 5th gen 4Runner. With that said, most vehicle manufacturers are constantly trying to improve their "cost of ownership" number so they can brag that their vehicles are "cheaper to own". One of the biggest methods they use is to increase the intervals of replacing fluids. Toyota even says that you can wait to change your motor oil every 10,000 miles..... LOL. Don't do this. I replace mine every 7,500 and it STILL comes out the color of tar.
Anyway, back to the transmission. Vehicle manufacturers will put a warranty on transmissions, basically based on how long the transmission will reliably operate before the fluid needs to be replaced. The problem with using this number yourself can be answered with two points:

1) The damage to a transmission from old fluid usually happens long before there are symptoms of the transmission failing. Think about smoking cigarettes for 10 years. You might not notice any health problems for the first 10 years of smoking! But does that mean that you can smoke for 10 years and you'll be just as healthy as someone who never smoked? Of course not.

2) After the warranty for a transmission expires, any damage that was not diagnosed and caught during the valid period of the warranty is now YOUR problem. But don't worry, Toyota will gladly charge you $7,500 for a new transmission plus $4,000 in labor! HAHAHAHAHAHAHAHA

Chances are, if you wait 100,000+ miles to touch your ATF, your transmission is already (to some degree) damaged.
I hope this answered all of your questions.