Much depends on initial build quality. Serious speakers like old Altecs, JBLs, Stephens, EVs, WEs, RCAs, Tannoys and such are going strong after 50 or more years.
I own Altec 605A monitors from the late 50s-early 60s that work fine and IMO sound better than 99% of the speakers discussed here. I recently owned some Altec 803 woofers made in 1957; I just gave them to a guy whose using them in JBL 4560 cabinets.
I just acquired an Altec 604 "Hollywood" monitor from the late 1940s, it works fine.
I know fellas that use RCA and Western Electric speakers from the 1930s and 40s.
However foam surrounds, even those used in high quality JBL, Lowther and Altec drivers, do rot. But JBL and Altec used such surrounds on only a few of their models.
On the subject of demagnetization: ferrite magnets are VERY stable. Alnico magnets are prone to demagnetization from overpowering; the strong fields around the voice-coil during overpowering will generally weaken an Alnico magnet. You can figure that ANY Alnico magnet speaker that needs rediaphragming has a weakened magnet.
One of the fellas at the Lansing Heritage site who has rebuilt many JBL Alnico drivers has NEVER measured the magnetic strength of a blown driver that the strength wasn't weakened.
If you really want to hear from fellas who know what they're talking about on this magnet subject and other subjects of interest concerning vintage speakers check the forum at the Lansing Heritage site, www.audioheritage.org