The "betterment" clause is a legitimate and routine procedure in the insurance industry. It is commonly used when the item or items that are being replaced under a covered loss are well used and nearing the end of their useful life. A hail-damaged roof on a home is a common replacement item that is often depreciated based on age and wear.
Joe Donnelly, a TDR writer, Ph. D. chemist, and extremely knowledgeable expert in all matters Cummins has written in TDR articles that he feels HPCR injectors have a service life of approximately 100k miles. That is his informed opinion based on his own research and testing. My own previous '06 was still running great and passed a dealer service department engine performance test at over 200k miles but my truck was highway driven, less than two years old, and had used only clean fuel and Fleetguard filters changed regularly. My injectors were certainly worn but were still okay. There is no guarantee that one of them might not have stuck open or failed closed at any time.
Cummins and Dodge engineers and service technicians probably have information indicating HPCR injectors do wear out at some average mileage. Actual service life probably varies with quality of fuel used, fuel filter service history, driving style, etc. but there is no doubt that a HPCR injector can and does wear out.
Your insurance company is not wrong. They are simply saying that with brand new injectors installed you would be in a better position than you were before your fuel system was contaminated. Stated another way they are depreciating your injectors by a percentage based on odometer miles and paying a portion of the repair cost.
Another legitimate way for the insurance company to settle the claim would be to pay for the installation of used injectors but used injectors would be difficult and impractical to find and purchase. They are doing the next best thing.
You can challenge your insurance company in court but you are not likely to win. They will have insurance industry lawyers and automobile service experts testify and you will almost certainly lose the case. Any honest attorney you consult will tell you the same thing I have told you.
An insurance company cannot and will not fully protect us from every misfortune that can occur.