an airline wants 200 people taking to Spain fifty times a week
A nice simple statement of requirement, indeed. Replace "Spain" with "countries of possible foreign policy interest between 2020 and 2060" and it starts to get a bit trickier. This highlights another problem with military procurement - the requirement often draws heavily upon intelligence and futurology, disciplines which make business forecasting appear a paragon of accuracy.
If we could be better at knowing precisely what we need our kit to do, we might be better at accepting OTS stuff that is "good enough" for the purpose. Unfortunately we have not proven very good at forecasting events over the years and I don't see any particular reason why that should change, so we'll probably just keep pushing the boundaries to insure against all those Rumsfeldian "unknowns".