The desire to have the most modern weapons is bumping up against the escalating cost of developing new technologies. Since the end of the Cold War, there is no one out there to compete with, but there is still the need to replace old equipment. There is also another, but unspoken, reason, and that is to keep friendly combat casualties as close to zero as possible. Equipment can be upgraded with new electronics and improved components. This has kept the B-52 flying, and successful, for over half a century. It's also possible to combine components from different weapons, add a few improvements, and have a new weapon. An example is the Sidewinder missile, that has been used to create an inexpensive anti-aircraft system for support ships. The RIM-116A Rolling Airframe Missile is basically a Sidewinder air-to-air missile (the most successful missile of it's class, in use since the late 1950s) with the infrared seeker of the Stinger shoulder fired anti-aircraft missile. The 162 pound RIM-116A has a 25 pound warhead and a five kilometer range. It doesn't require any guidance from ship radars. You just point and fire. The JDAM GPS guided bombs are another example of successful new weapons using existing technology. While you get more bang per buck with this approach, you also offend Congress, which depends on large defense contracts for patronage and getting reelected. So the larger, often wasteful, projects will keep coming, sometimes at the expense of cheaper and more effective weapons.