Those are some pretty good numbers. Throw in a couple of warranties and it would probably bring the average down to 17 years or so. Add some larger pumps and the average will drop even further to 12-14 years. More samples like 500 or 5,000 instead of 50, especially from other areas of the country will get you even closer to the 7 year design life for all submersibles.
But eliminating everything except the ones your company installed will probably up the average greatly to maybe 25 years or so. You should be proud of the fact that the professionals in your company are skilled enough to make most pumps last 3 times the national average. This is not all that uncommon as many professionals know how to reduce the cycling and get the most out of the equipment they have available.
Keeping up with changes in the industry is important. Pump manufacturers make changes without really announcing it. One day their pump works fine with Dole valves. The next thing you know impellers are melting down because they changed to a "floating impeller", which is really just a dragging impeller. These are cheaper to produce, but create much more heat and don't last as long as impellers held up by the motor thrust bearing. Professionals would figure these things out before long and switch to a different brand that will last.
Most professional installers know how to reduce cycling as much as possible to make a pump system last. They will be insistent on a large pressure tank to extend the pump run time to at least 1 minute, and will highly recommend twice that size tank for 2 minutes of run time, as it would cycle the pump even less. They will install frost free hydrants that can flow enough water to keep the pump from cycling off, or advise sprinkler systems be matched to the pump size. A patient installer will train his/her customer how to use enough water to keep the pump from cycling, how to recognize the symptoms of a waterlogged tank, and how to re-charge the tank with air when needed.
Besides checking the air in the pressure tank occasionally, the only thing a homeowner can do to extend the life of his/her pump system, is to make sure to use enough flow to keep the pump from cycling for long term uses of water. Getting the flow correct for heat pumps and sprinkler systems may take a little professional tuning. But garden hose irrigation takes a little training. If you have a 10 GPM pump and you are using 9 GPM or less the pump will cycle, which is not good. But if you use more than 12 GPM or so, the pressure will be too low. So the homeowner must be trained not to use too much water, yet still enough to keep the pump from cycling. A pump system installed by a professional, maintained and used properly by a homeowner with a little training, may last 3+ times longer than those installed by less qualified persons and given no maintenance.
This proves that what some people call the "old traditional system" with a large pressure tank only, when professionally installed and properly maintained can last an average of maybe 21-25 years. Only time will tell if adding a CSV to reduce the cycling even more will further extend the life of systems like these. The oldest CSV systems are only 24 years old now. But some of those are accelerated test on heavily used systems where the pump previously lasted only 5 to 10 years. If these accelerated tests are of any significance, we should also see greatly extended life of average pump systems as well.
Regardless of whether a CSV extends the life of a pump system or not, it can solve several major problems. The CSV eliminates any need for homeowner water use training. It will make the pump's flow rate match the amount of water being used, instead of the homeowner having to be responsible to always run the proper amount of water to keep the pump happy. As I said earlier, using the right amount of water for long terms is the only thing a homeowner can do to increase the life of his/her pump system. The CSV eliminates everyone of this responsibility, as it will make the pump automatically match any amount of water being used. This also eliminates the need for professional tuning to match heat pumps and irrigation systems.
The CSV can also save the pump from being destroyed by waterlogged tank conditions. First it completely eliminates cycling for long term uses of water, so the tank is less likely to lose it's air charge as there is no repetitive cycling action of the bladder. But if the tank does lose some or all of it's air charge, the CSV will just keep the pump running continuously and happily for as long as you are in the shower or using water. Sure until you replace the tank the pump will come on and go off instantly when a faucet is opened or closed, but it won't machinegun on/off 400 times WHILE you take a shower, as it would without a CSV.
Check valves, pressure switches, start capacitors, relays, tank bladders, even the wire slapping around down the well all last longer when you reduce the number of times they are used or cycled. With professional installation and proper maintenance the old pressure tank only systems can last a long time. Adding a CSV to a system with a small or large tank allows you to use water anyway you want without damage to the pump. Since professionals doing everything they can to limit the number of pump cycles makes pumps last a long time, the CSV is just another tool that can be used to further reduce cycling, along with all the problems that go with.