Betty,
There is a point here, the optimum platform may not be a big jet which tries to do everything. A platform which can be re-rolled quickly to be able to do the high impact but infrequent missions will be cheaper. Why carry an acoustic system, launchers sonobuoys etc on a maritime surveillance mission. If you stumble across a submarine you will be able to track it until it goes sinker. In peace time that should be enough. Remember we are now intelligence led! If there was a chance of stumbling across something the ac fit may be different.
The cost of trying to put everything into offers the ability to change role within a single mission, but in reality this does not happen often. So you do end up carrying around an elephant for the whole life of the platform.