It's true that the F/A-22 was designed to combat the old Soviet Union and that's no longer a threat. However, the sophistication of fighters built in other countries has continued to improve. Those countries don't show much hesitation against selling their planes to whomever has the money, so chances are we'll be facing them someday.

Since the end of WWII, the US military has followed a "treasure over blood" appropriations strategy where we'll pay for expensive weapons systems in order to reduce the casualty rates. This evolution has led us from the days when Sherman tanks (known to their crews as Ronsons - a type of cigarette lighter whose motto was "Lights every time") to the multimillion dollar Abrams that's damned near impervious to most antitank weapons. It's seen us evolve from planes like the B-17 and B-24 that took a crew of 9-10 men to deliver 2-3 tons of bombs to a target area (not necessarily the target) and could barely fly a few missions a week to a plane like the F-15E Strike Eagle that, with a crew of two, can attack multiple targets and fly multiple missions per day.

We're buying and maintaining fewer military airplanes, so the planes have to be the best we can afford. The F-15 is a great fighter with an RCS about the size of Mt. Rushmore (over 4 times bigger than a B-52 according to the linked reference) and based on a design that's over 30 years old. There's a reason why old designs tend to be replaced over time - it becomes too difficult to maintain and upgrade to meet changing technology and threats. The deciding factor for planes like the F/A-22 is stealth. Stealth is expensive, but it's also such an incredible advantage that it can't be overlooked. There's no way that you can make the F-15 stealthy.

Back in the 1970s when the F-15 was in flight test, the Air Force knew it couldn't afford as many of them as they wanted. As a result, they developed the "high/low" mix where a limited number of high end F-15s would be complemented by a larger number of lower end F-16s. The Navy did a similar thing with their mix of F-14s and F-18s. In both cases, the lower end planes were tasked primarily with air-to-ground missions. In the not too distant future, the same high/low strategy will be met by a limited number (<300) F/A-22s and a larger number of F-35s. All of the airframes will have stealth built in. For missions where stealth isn't required, external stores will allow for bigger weapons loads. If you check out pages 26-27 on the Nov 29, 2004 issue of Aviation Week, you'll see that the Air Force is evaluating future strike aircraft. One of the contendors is a FB-22 derivative of the F/A-22 design. It will be similar to the F-15E verses the F-15C. If they do this, the higher production rates should help lower the per unit F/A-22 costs.