[aceofspades19]

It's more that this is close to the best design possible for this mission, it would take decades and lots of money to develop another aircraft that does the same thing that might not produce that much of an advantage. There are lots of designs that have been around for ages because they are effective and there is no reason to replace them. Even in the civilian world, there are lots of aircraft types that have been around for 50+ years because there is no real reason to get rid of them as long as they can be upgraded to the latest systems.

It's really only when an aircraft can no longer be upgraded or its mission no longer exists and can't be adapted easily for another one is when it's retired.