"Vintage" speakers...good idea?

Over time, it is possible for the voice coil magnetic gap to shift or degrade as the "spider" ages, or for some paper cone drivers to dry out. However, a well-made and cared for driver can last decades and still sound close to new. As far as cabinetry goes, as long as joints haven't loosened, it should outlive you. Crossovers are another story, and you might encounter a dried out cap on occasion with really old specimens. Determining the average life expectancy of a speaker is really going to be determined by how well the unit was originally built - something that varies widely even today.

One last caveat I might add regarding vintage or classic speakers is that upgrading components with modern replacement parts could reduce their resale value. I have seen the effects of this practice with some otherwise vintage tube gear, where modern caps and wires had been substituted wholesale for the original, still working, classic parts, changing but not necessarily improving the voice of the gear - and seriously impairing its market value. Personally, I appreciate keeping true classic gear close to original (it would be like installing a breakerless distributor in an old AC Cobra), but if that is not an issue to you, by all means modify. However, a fifty-year old K-horn with Cardas binding posts, Hovlands and silver Alpha core inside won't fetch as much as a good working original at resale time, IMHO.