Battery manufacturers do not build to "obsolescence" so I really do not understand how that "dealer" could spout that.
No where did I say that battery builders build to obsolescence. Re read my post please. They build batteries knowing that most will have a working life of 3 - 5 years and few warranty anything more than a prorate after the first year or two. How they are used in the real world varies greatly and that affects the working life.
They build the battery to PROVIDE a certain amount of energy and YOU pay for the "warranty period" buy paying a "premium price" from that brand (in your case Trojan and they are not known for low cost batteries because they warranty the batteries for a longer period than most).
Yep, see my comment above... 3 - 5 years is about all they expect the average consumer to get.
Bring your "Arizona" battery up north for summer and winter time.. You think Arizona heat is hard on batteries, try on some summer heat of upper 90's then plunge below zero for winter for weeks on end.
Like I said weather is a factor, no disagreement that your climate is tough on batteries. I have started my 1995 LTD with 187,000 miles up in Angel Fire when it was 14 degrees below zero and at 115 back home.
If all you get is 3 yrs out of a starting battery, there IS something wrong with your vehicles or how you use them.
If that were true, and it is not, why does almost every new car manufacturer recommend/require that the battery be replaced in or will no longer warranty it after 3 years? My 2019
Honda Ridgeline sat in the AZ sun for about a year before I took delivery and the battery crapped out in 4 days. The manager of the dealership along with a couple of service advisors all agreed that this was not unusual due to the heat they are subjected to here in southern Arizona.
My DW has a very short commute, 11 miles round trip, THAT is hard on batteries, vehicle NEVER really gets the battery fully charged after each start. Even then, typically am able to get 5-6yrs out of the batteries in her vehicle. They are consistently under charged. You really need 30 minutes of driving at moderate speeds to put back the energy that was pulled when you started the vehicle. Short trips, stop and go traffic and use of a lot of accessories can easily prevent your vehicle from fully recharging the battery.
Like I said the life of any battery is largely contingent on how it is used and/or abused along with weather which we also agree with. All of my starting batteries {I own 5 vehicles} get checked on a regular basis and I have never had one show anything but fully charged until they moved into the 4th or 5th year.
Contrast that to my commute of an hr each way, the worst battery life I have gotten out of vehicle batteries has been 9 yrs, the best has been 13 yrs! And yes, I drive and keep vehicles for a long time, the highest mileage one was 240,000 miles and 16 yrs old when I sold it, it had exactly ONE battery replacement. By the way, that 13 yrs WAS with the ORIGINAL OEM battery!
If you want to say so it is OK with me but in your original post you make the blanket statement that everyone should get 9-10 years out of their starting batteries and that is absurd. Then you tell me that 3 - 5 years from a starting battery indicates there is something wrong with my electrical system which is also absurd. My post is intended for the benefit of those folks out there that don't know any better.
The life you get out any battery highly depends on the depth of discharge and how fast you FULLY recharge it, not so much with brand name or just how much you paid for that brand name. In other words, if you care for it, it will treat you with a long life.
Now you are confusing deep cycle batteries with starting batteries which are two totally different animals with distinctly different functions. Starting batteries use a relatively small amount of energy for an extremely brief period of time. They are designed to do this over and over again. You do not need to drive 30 minutes to replace the energy expended in a simple start, the truth is about 5 minutes or less will easily do that unless there is something seriously wrong with your vehicle. You can pull a starting battery down flat and 30 minutes of driving will replace the bulk of that expenditure but... sadly a starting battery will only survive this sort of abuse 2 or perhaps 3 times and then it is toast.
Abuse the battery and it treats you to a very short life.
Once again we agree but you need to keep your expectations in the real world recognizing that there are a host of factors in play.
As always... opinions and YMMV
:)