How long does it take a decent quality speaker to "wear out"?

Durability and lifetime depends on specific design.  I have heard speakers with 70 year old drivers that sound terrific.  Paper woofers with pleated fabric surrounds can be quite durable compared to even modern woofers with rubber surrounds.  My midrange compression drivers are about 80 years old. Even speakers with surrounds that go bad can be long-lived if you accept that surrounds have to be replaced every twenty years or so.

The life expectancy of electrostatic panels depend on the specific models and operating conditions.  The Martin Logan panels do not arc and they do not develop holes in the diaphragm from arcing.  In that respect the panels are MUCH more durable than are the panels on Quad speakers.  However, the Martin Logan panels can be susceptible to to the wires that bring the charge to the panels corroding at the point where the wires attach to the panel.  I know someone who suffered from this problem because he stored his speakers in a garage that gets quite humid in the summer.  Owners in humid Southeast Asian countries also have reported similar problems with Martin Logan panels.  Otherwise, the Martin Logan panels seem to be quite long-lived.