Thoughts on extending speaker life.

Age can deteriorate any part, cone, surround, you could burn a voice coil, ....

I have speakers with drivers made in 1956. The tweeter horns and mid-range horns appear to be bulletproof, 63 years old, still sound terrific. I do have spare drivers waiting (as someone else said, they were made in 1956 also, they just haven't done any work).

The 15" woofers (paper with flexible pleated cloth surrounds), this one

https://reverb.com/item/2307415-electro-voice-model-15w-alnico-magnet-15-woofer-new-in-original-box-nos

I had them re-coned professionally many years ago, have a spare waiting, and re-coned one myself not long ago.

Videos show how to do it, cost is minimal, the hardest part is scraping the old glue off the spider ring.

http://www.speakerworks.com/speaker-repair-kits-s/65.htm?gclid=Cj0KCQjw3uboBRDCARIsAO2XcYCQUY1lUVYTubmOssOLwtflmQlsxhhxHHnNrms9nKalZrIaNGzk2ngaAqehEALw_wcB

Based on the success, I re-coned my Velodyne 12" sub, it went well also.

I used to think about 'sag', especially such advanced years, but there is no evidence in mine. The voice coil would rub, you would hear it for sure.

I should have said, 

15" I replaced the coil, cone, surround

12" I just replaced the surround, the cone and coil were fine, and getting the voice coil centered was easy after watching the video.

too many.

I have saved 'great' stuff for so long, moving it out now bit by bit

physically don't want to pick up anything heavy (bad back, bad knees, bad attitude).

take up too much space

price drops to nothing for most, better to sell when there is more to get out of them.

we are more likely to move on to something 'new to us' than go back.

you know what you will NEVER part with, off with the rest!

IF the suspension 'sagged', the coil that drives the cone would rub in the magnetic gap, and you would hear it. Never experienced it, never heard of anyone ever experiencing it.

I had a pro re-cone one of my 15" woofer off center. Got it home, mounted, play, what is that?

I had a staple holding interior insulation vibrate loose, fall onto the back of the 15" cone when they were down firing, a weird random sound, you would not think would come from the woofer, what the hell is that. A Sherlock Holmes investigation, I felt so lucky when I found the little bugger hiding under a spider support. 

foam dissolving (resultant permanent sag-off centering), paper cones drying out, cones developing holes, losing acoustic seal, these happen.

If you can get to them, checking, tightening your drivers, especially big ones, is smart  Rotating at that time, seems innocent enough, but, there is potential to compromise the seal at the gasket, so be aware, verify ok, or put back as it was.My 15" woofers, and bigger midrange horns can get loose. Originally 4 clamps/screws, I added a few extra when making new enclosures.

Verify woofer seal for your confidence the problem does not exist: turn your ac system fan off, close the windows and doors, take the grill off, Mickey Hart, volume up, light a match (we used to use cigarette smoke), hold the match as you circle the perimeter, oh happy or oh sad day. Ports: stuff them for the test. While stuffed, listen a while, hear the difference, then open em up, listen again.