OK. For those who want to learn, here we go.
Here is a link to the patent for a sealed transmission breather. This will explain why a sealed transmission has a breather and how it prevents or minimizes oxidation of the transmission fluid. US5129422A - Transmission breather control valve - Google Patents
Here is one from the society of automotive engineers (SAE) also about breathers on sealed transmissions, and although you have to pay to read this one, you can read the summary and get the idea of what they are talking about. Transmission Air Breathing Suppressor (TABS) Valve - A Device for Improving Automatic Transmission Fluid Life
Here are a series of 5 history of transmission fluid videos done by Prof John Kelly who is the most knowledgeable transmission nerd I know of. In part 4 of the history of Chrysler Trans Fluid he talks about the discovery of sealed transmission fluid longevity and explains why preventing people from checking the fluid and adding fluid and exposure to oxygen are all bad and led manufactures to eliminate anyone being able to check or change fluid easily. He talks about doing a lifetime fluid video, but it does not look like he every finished it. If anyone can find it, drop a link because I would love to watch that.
This is a good article that has a section that talks about lifetime fluids. It has a good history of transmission fluid and talks about the original discovery of hermetically sealing fluid in the transmission by Chrysler . https://everipedia.org/wiki/lang_en/Automatic_transmission_fluid
A couple more things:
I don't care one way or the other if you change your transmission fluid. I can only explain the engineering behind why manufactures are saying that under normal driving conditions you don't have to do that. You do what makes you feel good.
I am not going to explain the math. It's Jr high school level chemistry and algebra.
For those who want to argue against Toyota's recommendation to not change the trans fluid, I ask what is Toyota's motivation? Are they trying to boost their 15 year/300,000 miles transmission sales? Why would they want to sell less transmission fluid and why would the dealers want to do less maintenance work? And why would they want to purposefully shorten the life expectancy of their trucks over transmission fluid?
I think the answer is that they think their transmissions on average will last 3-400K miles without any service, and even when the transmissions does fail, it will probably not be because of the fluid. And that happens to be my definition of lifetime as well, so I don't plan to do anything more than a pan drop and filter change every 125-150K.
The best of the best to ya all!