I just had a conversation with a customer about the "lifetime transmission fluid" in their Toyota. They have about 40,000 miles on their Prius and the ATF is black and burnt. The transmissions of today are fairly complex inside, and hybrid vehicles typically have high voltage electric motors in them. The goal of fluid maintenance is to keep the quality of the lubricating oil/fluid as clean and debris free as possible.
Most vehicles now DO NOT have a serviceable filter inside the transmission, making it even more crucial to keep the fluid clean. The cost to include fluid servicing in a factory scheduled maintenance is relatively inexpensive. What it does for you is keep the burnt and dirty fluid that is naturally starting to occur to a minimum. It does not all not come out of the transmission at once, it is typically only going to be a slight dilution at each fluid change interval. You can maybe see how if it is allowed to get dark and burnt, it is harder to get it to a clean state again. I use the analogy of having a giant bowl of chocolate pudding (because I like it), and eating a small amount out of it. Then, add vanilla pudding to replace what you ate and mix. It most certainly did not turn to vanilla yet. It will take a lot more eating. The better way is to start with vanilla, and keep it vanilla (even though chocolate is awesome).
At Toole's Garage, we recommend transmission fluid replacement at least every 30,000 miles, and even every 15,000 miles on some makes. How and where you drive determines how dirty it gets as well.
So, what does "lifetime fluid" mean? It literally means, "Lifetime of the component". Well.....how long do you want YOUR transmission to last? :) We would like to never have to replace it.
Hope this was helpful!
Dave "The Car Guy" Toole