I've always read "lifetime" from automotive manufacturers as "lifetime of the warranty period"
My view which is shared by many is "lifetime" means the fluid will last the life of the tranny. When the tranny fails it's reached its end of life regardless of miles.
:lol:
I'm at 197k and just do a drain and fill now and then to add some fresh fluid while removing some old. It's simple, inexpensive and pretty much fail-proof. I wheel hard, tow a trailer and don't believe in risking any expensive parts by doing what really doesn't need to be done.