It is impossible to design a vehicle with every situation in mind. You design for the most common task and modify them as needed to adapt or optimize them for specific situations. Else you'll get a piece of equipment that performs poorly in every situation.
And that's true with a lot of things. Not only military vehicles.
Adapting a "general purpose" vehicle for specific condition of operations can be done very easily if the vehicle is well designed. That being said, your money (and time) would be put to much better use in designing a vehicle easy (and cheap) to adapt for various conditions.
Here's a quick example. You could have helicopters that are used in various operations around the world. Now, if you operate them in Middle east, you might need special air filters at the intakes for the jet engines (jet engines do not like sand
). On the other hand, this kind of filters is expensive and reduces the efficiency of the engine, raising fuel consumption (and operation costs). For this reason, you do not want to use filters like that in situations where you do not need them (e.g. no sand in the air). If you construct them with filters built-in, you'll be adapted for Middle East operation, but if you have to use it in the Balkans, operation are going to be very expensive for no reasons. Worse, in icing conditions, you might get other type of problems or even risk loosing engines, grounding your fleet. On the other hand, if you have an helicopter with no filters, it will work pretty well everywhere, but you'll have major troubles in the middle east, with loss of engines and rapid aging of all mechanical parts.
The best solution would be to have a helicopter where the filters can be removed or placed in depending the current situation. If you have a design where you can do that, you'll have a fleet 100% operational in every condition. Agreed, the air filters that are "removable" might be less effective, but still more than none and removing them would sure be better than leaving them on all the time.
I do get your point though, but I only partly agree with it. The process of design such a vehicle is not an easy task. One of my colleagues is part of the team in charge of upgrading the CAF F18 to current standards. That gave me a little insight on that kind of projects and the complexity of it is impressive. There a lot of choices and decision to make and it is not surprising that during the process some adjustment needs to be done. To me, the fact that adjustments are made is even a good sign, not a sign that somebody wasn't doing his job.
Also, production wise, it's not always possible to hold a production line to make adjustment to it while it's on the run. One side you are pushing to make changes, which take time and adjustment to the production of the products (and if it needs changes to the already finished products, it's even worse), but, often, you cannot afford to push back the production deadline (for whether reason).