I'll throw in my personal experience with MANY cars.
I flip used cars as a hobby / extra income. I come across A TON of cars that very obviously still have the original Lube in the tranny. The car shifts poorly and rough.
After a fluid change, the transmission INSTANTLY starts shifting better. Within 100 miles of use, it gets even better as the fluid cleans dirty parts and provides better Lubrication for better operation / performance. I then usually do another mini flush to insure extra clean and fresh Lube in the transmission for the new owner.
I personally feel the whole lifetime Lube movement is probably an effort to save oil and or lower the cost of maintenance for the ORIGINAL BUYER of the car while still under warranty.
Like someone else said earlier, most 10-15 year old cars with 150-200k miles are completely destroyed inside and out and need to be crushed / recycled because they are nasty, dirty and worn out rust buckets. I can see the original transmission Lube providing just enough protection to keep the transmission going in one piece just long enough to survive the use before it heads to the junk yard.
If 90% of the Millions of average daily beaters have lifetime Lube that nobody touches and then the cars go to the junk yard, technically... no harm no foul, right? The car is toast, interior is worn out, engine is leaking and burning oil and the original transmission fluid is also done and burnt to a crisp and will cause the transmission to fail soon.
Now, if you're trying to keep the car in 100% OEM spec ( LIKE NEW ) then no way can the word "Lifetime" be associated with your maintenance schedule. That fluid needs to go after 60-100k in order to keep the vehicle realiable, and that's not even considering the smoothness of the transmission operation. I personally prefer my cars to shift smoothly. Jerking and rough shifting is a NO GO to me.
I have 50k miles on my 2014 Mazda 3 and I've changed the transmission oil in the 6 speed manual twice already leading up to this point. I flushed the original fill after a few thousand miles of use and did another easy drain and fill at 30k miles. Both times the gears shifted MUCH smoother with fresh, new oil providing PERFECT operation of the gear synchronizers. Recently wifey said she noticed the gears getting crunchy again, so that's my que that another flush is needed to remove the metal shavings out of the system to allow for smooth operation again.
Our Mazda 3 is modified and dyno tuned. The transmission also has a shift shifter and stiffer bushings / engine mounts so I can FEEL the gears sliding into place. Once you get used to how it's suppose to feel when NEW, I swear I can feel the SLIGHTEST difference and degradation in shift quality, so to me personally, it's a no brainer that if you want 100% OEM spec, new fluids are needed throughout the drivetrain.
Those are my thoughts and personal experience with HUNDREDS of cars that I service regularly. Feedback from owners who leave my shop after a transmission flush and report saying "wow, it shifts so much smoother now" is all I need to know in regards to this subject.
Just my thoughts.