What causes a switch to reach the end of its lifetime?
In general, the long-term problem is degredation of the electrical contacts within the switch. This happens as a result of the two different types of actions which occur during operation.

1) Closing the switch. If the load has a large capacitance, there will be a large surge current through the contacts when they are closed. In the worst case, the contacts can be welded together, but even if that doesn't happen, the contact area can become pitted and unreliable.

2) Opening the switch. If the load has a large inductance, an arc will form as the load's magnetic field collapses. This will erode one contact, depending on polarity, and deposit material on the other. Eventually, the two contacts will become so damaged that the switch will fail to make contact.

And, just to complicate matters, it is necessary to specify the current levels which will be found in operation. The normal contact material in switches is silver. This has many good qualities, but silver is susceptible to corrosion, especially tarnishing due to traces of sulphur in the air. For large currents, silver contacts will usually show self-cleaning behavior as contact is made, but this may not occur at low currents - typically a couple of hundred milliamps or less. Switches intended for low-level operation usually have gold flashing on the contacts, which will not tarnish. However, such switches need special care. Specifically, they must not be used both for high and low level switching, since high currents will burn off the gold flashing, which is usually 100 microinch or less.