It isn't uncommon for satellites to last well past their expected lifespan.

I used to operate DSCS-III military communications satellites. They were designed for a 10 year operational life. Recently, I read where two of them launched in 1985 are still operational. An old Navy FleetSat commsat lasted for over 20 years but was designed to only last for 7. I've heard that some of the old Navy Transit navigation satellites have lasted for well over 20 years. GPS satellites were designed to last for 7 years, but many of them last much longer. This is a good thing.

Of course, they all have a ways to go to match the old BUFF for longevity. While it isn't a satellite, it has lasted decades longer than originally projected when they were designed. Not only are all of them older than their crews (the newest one built rolled off the line 42 years ago), but some of them are on their 3rd generation of crews. Their grandfathers flew them in the 1950s, their fathers flew them in the 1970s and 80s, and they're still flying today. They may be in service for another 30 years.