Oh come now, how can you possibly contend that your Betters will rush a decision such as this?
I would note, though, that one of the main drivers for changing requirements is the fact the MOST significant military purchases are for state-of-the-art equipment which is still a twinkle in someone's eye when the specifications are first written. The contractors will come back with their proposals and bids, but both sides recognize that research and development, which is what these procurements really are, is subject to major changes during its run. Some changes will be for the better - research will find a way to meet the specs which costs less or provides FAR BETTER performance than the initial requirement. But most often things will not go as well as estimated and performance or cost will suffer. And if there is a MAJOR breakthrough during the R&D activity, the specifications may be rewritten to take advantage of that in hopes of make the new toys be usable for a longer period. For while there ARE worthless POS whose only concern is how much loot is in it for them or their buddies, MOST of those involved in the procurement process really do want to get equipment that will best protect the lives of the military who will be using it.
That is the theory ... but of the RAF had bought F-16s instead of Eurofighter, it would have had a workable fighter-bomber for very much less money. But no, we had to support European co-operative projects. Even without the Nimrod, the UAV programme and and a few others, we are still looking at an over-spend of well over £20 billion, with not one of the problems solvable by giving the actual purchasing element to Tesco.
Part of the problem you identify, though, is the "nothing but the best" syndrome. This very often means late, over-priced, with limited capabilities because they had to be cut back for cost reasons, and fewer than we need. The driving phrases need to be "good enough" and "the bird in the hand is worth two super-eagles in the stratosphere".
It is a hell of a road that is paved with good intentions. It never ends up going where you want.