Every time the transmission shifts it's one less your getting down the road, brake pads are cheap (and last forever). If I don't let my truck downshift 2-3 times down every hill, that saves my transmission hundreds to thousands of shifts per year. Manual mode works well for this.
Your first statement is completely false. A transmission does not have some magic number of shifts before it wears out. Although brake pads may be cheap, if you live somewhere with real hills and tow much you know that brakes will overheat and rotors will warp when over used. Manual mode can actually be harder on a transmission than letting the system choose when to shift. Human drivers shift when it 'sounds good' and not when the engine torque, rpm, line pressure, etc. are best for the shift. This has been documented by many of the manufacturers and is why some of them do their 'manual' mode differently and let the computer override the driver much more.
Have you ever driven a GM 6 speed? I think you need to look up the definitions for elegant and refined.
I drive one every day. The 6L45 is elegant and refined enough for BMW to buy them in quantity from GM for multiple vehicle models. Have you ever diagnosed and repaired automatic transmissions as a technician?
My guess is that your comments have to do with not liking the shift pattern of the transmission on the V6. I was speaking of the design, durability and mechanicals of the transmission. What you experience is what happens when you mismatch a drivetrain. That engine does not have the right torque curve to be in a truck and that causes the transmission to shift a lot to keep in the power band. That is one of the reasons that I didn't buy the V6. It is looking like they are bringing a new V6 tuned properly for a truck in the 2017. Unfortunately it will have the, as of yet, somewhat unproven 8 speed transmission with it. If you have a chance to drive a diesel in front of the same transmission you will find that it is a COMPLETELY different experience. Smooth as butter and nice confident shifts even while loaded near maximum capacity. After having worked on the predecessors of this transmission, I would take the 6L50 any day over them (5L50, 4L60/700R4). It is truly much stronger, more efficient and better designed.
Growing up and living in a 'hilly' area with plenty of 6-8% grades, and even higher on township roads, I have always used downshifting on hills even in daily driving of automatics. I usually buy vehicles with well over 100k on the clock and run them till there is too much rust to bother fixing. All of the transmission issues I have experienced in the past 25 years have been a common failure mode of that particular model and completely unrelated to engine braking. I have also never wore clutch sets to the point of causing slipping in any of those transmissions.
When it comes to customer transmissions, I would always see the higher mileage known failure modes. When transmissions died early it could be traced to improper use (abuse) or maintenance neglect pretty much every time. I can count on one hand, how many were a defect failure, and those all failed within six months of driving off the lot when new. I was not around for the 'bad old days' of the early to mid '80s transmissions and thus have only heard the stories and read the bedtime books.
YMMV, but I will continue to happily use the exhaust brake on my 2.8L Duramax whether loaded or not when descending hills