Throwing expensive toys away if they work is bit silly, that's why phasing operational stuff out takes so long. Especially if you account for the need to retrain all your people, not just the pilots, but service crew as well.
That goes for all the other major Western fighters too. The ONLY reason we're buying Eurofighter Typhoons is contractual obligation. Tornadoes (more-or-less a British equivalent to the Tomcat) are still perfectly good. If there were worries about the airframes we should have simply spun the production lines back up and made more of the proven design.
When you compare it to other larger military aircraft, it doesn't seem very long at all. The B52 has been in active service since 1955. The A-10 was introduced in 1977 and isn't expected to be replaced until nearly 2030.
Grew up about 5 miles from the plant in Marietta Ga.