How long should high quality speakers last?

good OP! 

speakers need to check periodically(3..10yr), for: cabinets integrity, internal connections deox, bipolar electrolytic capacitors values, inductors, damping material etc..

simplest test is a slow sweep 10Hz..25kHz, starting at low loudness, then gradually increase volume to the max. you will be surprised to hear resonances and distortions caused by drivers and cabinets (cracks in adhesives, lose panels and components). another discovery will be many resonating objects in the listening room (paintings on the walls, glasses etc). 

From personal experience these kinds of questions only arise when someone is straining to justify a big purchase given the available budget. Sure, money management is a critical life-skill but guess what, life is short and other than passing on generational wealth, live life to your fullest. So the short answer is; loudspeakers last plenty of years-worry about your loved ones and your liver, lungs, and heart before worrying about how long your dang loudspeakers will last. 

Bought a used pair of Dali helicon 800s from this site in 2005, still sound fantastic. Had one driver repair in all that time, will never get rid of them.

These are close to 50 years old and still sound pretty damn good!

https://youtu.be/iGa9259pz-I