Speakers have two vulnerabilities.
#1 biggest is the deterioration of the surround material on the larger drive units. IF the material is either paper, only on very old (1950's) types, or real rubber, then the surrounds are good for at minimum 20.30 years.. For the unfortunate with foam surrounds, the lifespan is typically 8/12 years.. sooner if speakers are used hard, longer if used gently, but foam rots from the ozone in the air and not as much from usage. Foam surrounds are very soft and easily snagged or torn.
foam surrounds are easy to replace, and many kits are available to replace foam surrounds. Expect to pay $100/150 for a four surround do it yourself kit. You need only to have minimum handyman skills to replace foam surrounds.
For rubber and paper, the speakers need reconing, a more labor intensive job best left to professionals. Rubber is usually black roll, and flexible, hard to tear, and pretty good, better speakers almost all use rubbler now. Paper surrounds were on older speakers from the 40's and 50's.. paper lasts nearly forever. but is also the hardest to have fixed properly if damaged. A real expert is needed to fix them.
The way to tell if your surrounds are bad is to look at the surface of the cones, The cones have a surround, Is it intact? Yes then no problem. Do not replace them until they are broken or cracked. (If a crack allows air to bypass the cone, it is broken, no matter how small the crack or tear.) Do not get surrounds fixed as a 'preventatative measure. One never knows how long a surround may last, and a repair may fail before the original would have.. so if it is not broken DO NOT FIX IT.
The second weak spot in a speaker is the crossover in the capacitors. Capacitors usually last about 20 years, and again, the problem is inherent in the design, and use or non use in a speaker does not matter too much. If the speaker does not play correctly, and it is not a scratching sound, and the surrounds are not broken or ripped, then it is invariably the capacitors in a crossover.
Replacing all the crossover caps after 20 years as a preventative measure is a reasonable idea.
As for the rest, a speaker can last 70 years, easily without a problem..even longer.
A scratcing sound indicates a speaker voice coil is damaged. This involves the same cost as a reconing. Voice coils become damaged from extreme hard use. So I do not think you would ever have this problem.