There is actually a fairly lucrative market for purpose built emergency service vehicles. Well lucrative if the government actually did the smart thing and invested in them!
I.e. design and build a chassis and cab with all the fitting and equipment needed. Make the engine and running gear easy to swap in and out but keep everything else... It's what the military do with their vehicles and aircraft. So while the initial investment might be more, to replace running gear at less than half the cost of replacing the whole car, in the long term makes far more sense.
And as time goes by, technology can be upgraded rather then needing new... I.e. upgrade cameras, sensors, electronics etc.
I'm actually surprised the Carbon Motors experiment in the USA failed... It makes sense to have something purpose built.
Same goes for ambulances really. And I don't mean the failed LWB Holden Commodore ute experiment. I mean a mobile intensive care "pod" on the back of a light truck chassis.