system
Given the following post, respond as if you're commenting on Reddit:
human
How is it possible that satellites last far longer than planned?

For example, the Meteosat-7 satellite was launched around 20 years ago and is still working fine without the need for maintenance. The satellite was supposed to stay in geostationary orbit for only 6 years.
gpt
Usually when engineers say something will last for X years, it means that the thing has 95% (for example) chance of still working after X years. It doesn't mean that as soon as the X years have passed the satellite will stop working. This is something people often don't understand when they are talking about planned obsolescence. 

For satellites there are a couple of limitations on the lifespan on top of stuff just breaking up. First is the propulsion. In geostationary orbit you need a bit of propellant to correct for perturbations caused by the moon and the sun gravitational pull as well light pressure. When the propellant runs out the satellites are placed into a graveyard orbit so that they are out of the way of other satellites. Sometime thrusters perform better than expected, or the rocket is extra precise in its launch so you end up having spare propellant and you can extend the mission. 

Another thing that happens as satellites age is the degradation of solar panels and batteries. The solar panels lose efficiency when they are struck by radiation and cosmic rays.

Lastly sometime the mission duration is tied to the money needed to operate a satellite. To extend a mission you need money to pay the people in charge of the mission. The ground equipment can be very expensive too.