Its called managing your fleet !!
Look at an interstate trucking company, do they put their brand new trucks on local, no. They gradually donwgrade the trucks onot intrastate , then local then sell them.
A fire service is no different. A brand new truck sitting in a less than 10 call a year brigade isn't managing your fleet. A 5-10 year old truck that is in perfect working order will do fine for those 10 calls, and that truck will have been worn in and perhaps have the bugs worked out. Gone are the days of councils buying new trucks every 20 years, and look at it this way, I doubt many people will keep a truck for 20 years anymore, they might get it when its 10 and keep it until its 20, and those that might be doing 250 calls a year plus might only keep a truck 5 years (ok morphett vale excepted).
You have to look at it from the big picture, which I myself probably don't do enough. This way we can try and keep on top of a rapidly aging fleet and get the best value for the large capital outlay.
There are some areas that pull the old political line and get their local polly to get them a new truck.....I have seen it happen.