Unfortunately, the spectrum of military operations is broad and constantly changing. In the 1980s, military forces in the Western world and the Warsaw Pact were structured for large-scale, force-on-force combat. The Gulf War in 1991 provided such a start example of the superiority of US conventional forces that few countries today would seek to risk a conflict on those terms. So now we have asymmetric warfare involving non-state actors. That change has major consequences for military procurement - reducing need for main battle tanks and traditional ISR and more need for flexible, mobile infantry and special forces, as well as persistent surveillance to identify hostiles (which is, itself, hard). All of these changes hit following the invasion of Afghanistan in 2001, in other words well within the past 15 years. One could argue that most of the changes to combat conditions happened as soon as the initial objective of removing the Taliban from power was achieved - the nature of the combat changed within a matter of weeks not years.
Your comparison with commercial space exploration attempts is, unfortunately, not valid. The conditions of space exploration are well-known and they don't change. It's simply a question of money and commercial interest to determine what type(s) of space vehicles will be commercially viable. The requirements don't change much over time - if you want the average Joe (or Josephine) to get into space then you need a vehicle with a carrying capacity that can make money. Technology impacts your ability to do that but designs aren't subject to wildly swinging requirements changes.
