If one treasures cabinetry this is also an area to be aware of, as veneers, if not properly glued/pressed, can detach from the particle board (or the more rarely used plywood) over the gears, as can joints, and solid wood, if not properly dried, can "work" itself to create cracks or otherwise bend out of shape. Cross-over parts such as electrolytic capacitors dry out after some years. As already mentioned different surround materials deteriorate over years, some more rapidly than others, also depending on atmospheric conditions such as humidity and shift in temperatures, and direct sunlight is never a friend of most any driver. It seems treated cloth surrounds lasts for decades, and where used in horns and hidden away are often regarded as mint-condition young drivers even after 25 years. True heirloom speakers that combine excellent cabinetry and overall craftsmanship with quality drivers and cross-over components, and that are used carefully under "normal" conditions, should last many decades without sound degradation, as I'm sure quality speakers that do not aspire to "heirloom"-status should easily last 25 years or more, if properly treated.
