How long should high quality speakers last?

More modern, polyether-based foam surrounds are very durable, and when treated with a "foam guard" should last beyond 25 years. In any event direct sunlight is best prevented from reaching exposed cones to extend longevity of the surrounds and avoid potential discolouring of the cones. When speaking of foam surrounds it seems cone movement can be a co-contributor to deterioration, likely only with older polyurethane types.

Compression drivers, as stated by @larryi, are very long lasting. Some older compression drivers with aluminium diaphragms and integrated surrounds were marred by metal fatique when crossed low (like with high output cinema use and crossed at 500Hz), a problem that was avoided with the advent of titanium diaphragms. However in a home setting compression drivers should see neither thermal nor mechanical malfunction for a virtually unlimited lifespan. 

Indeed many older, especially pro-oriented drivers have proven their worth even after several decades of use, refurbished or not. Either way it demonstrates they are very high quality designs that easily stand the test of time, in quite a few cases in a superior fashion over modern day equivalents. 

Jeff Days’ vintage office system:

These are heavy loudspeakers, somewhere in the 250 pound range each as a guesstimate. These vintage loudspeakers—like me, are over six decades old now—have accumulated a few scuffs and scrapes over the years, but still they are in very nice cosmetic condition overall considering their age, and the drivers, horns, crossovers and cabinets are in essentially perfect operational condition—a testimony to the impeccable quality of Altec products from that period.

@unreceivedogma wrote:

Did I say “sound the same as new”? 

I did not. 

However, I obviously would not still be using them if I was not getting anything less than outstanding performance from them. 

It appears the post you were replying to has been deleted, but nonetheless I'd say you would definitely be in your right to claim your vintage speakers sound "like new," also being they have been serviced regularly. Who really, from actual experience, refers to and knows about the sonic difference between more or less brand new, properly run in Altec's and their decades old, well-kept equivalent? Demagnetization of Alnico magnets in particular, as has been touched upon, is really only an issue with excessive heating of the magnet from large voice coils or shock treatment, and any natural degradation of magnets is hardly anything other than academic in nature.

The real takeaway here is that such drivers, even decades old, are so transiently alive, dynamic and natural sounding that many newer, lower efficiency modern drivers seem stale and lifeless by comparison. Speaking of sonic detrimements due to aging here misses not only its relevance but also fails to take into consideration an unbiased assessment of these older designs and their merits.