How long do speakers last?

@mucker , The sky is the limit with fabric just please do not do the paisley.  You will need a person competent with a sowing machine. The sock has to be made undersized so it will stretch tight putting it on. Think of putting a girdle on an elephant. 

One of the safest bets in old speakers are Magnepans. No voice coils to burn out, no surrounds to dry up and disintegrate, no fancy cabinetry to get banged up. There is really very little to damage or go wrong in a Maggie but you do have to be careful around that ribbon tweeter. 

Can you define last?   Old speakers still play and sound good, but over time the sound quality, assuming no damage, will degrade slowly.  I had a 20 year old pair of SF and they sound good, but not as good as new.  Difficult to really compare with the passage of time.  I was told the caps (or other elements) in the cross overs can go out of spec after decades and the aging rubber or what ever it is can break down a bit.  

@mijostyn "Think of putting a girdle on an elephant." Thank you for that visual! : )

I have a pair of speakers that are 30 yo. Nothing has been done to them and they still sound outstanding. If they look good and sound good, I would take the chance. You can always resell for minimal loss. Some times you gotta take a chance!