t43562
For many reasons new combat aircraft are not fully capable when they first enter service. 20 years later they start to look seriously useful bits of kit.
Don't take my word for it just look at the history of any type that first entered service since say 1970.
After 40 or so years of service various factors start to make the type look as if it has peaked and so should be replaced before too long. In my view 20 to 40 are likely to be the best operational years.