Speakers continue to break in over a long period of time. Within 200 hours of playing they should be within the last 2-3% in my tests (200 hours at stage volumes for brands other than mine, I must add).
If you variac them (part of my FBI service), you get most of it done to within 90-95%. However, one thing the variac won't do is force the speaker cone to it's fullest excursion. Only high volume playing will do that as it sends a big signal to the speaker from a stop position and makes it extend further when you hit a power chord with volume. There's no way to do that with each speaker without actually playing it, or going through an extensive loop that goes at 30-40w in open air, which also takes awhile.
So the speaker will continue to break in very slightly after a variac break in.
As for speakers wearing out...sure, they wear out, but you should be able to get 25-30 years out of them if you don't do stupid things (i.e. drop the cab, stab the speaker ala Jimi or Pete Townsend, or like danyeo and put your foot through them). If you purposely crank up your 100w, dime the amp (that means putting the controls on 10) and only hook up 1/2 of your cab cuz you were bombed and missed the mono input and landed in one of the stereo jacks (meaning only 1/2 your speakers of the 4x12 are hooked up) you could cook some speakers quickly.
I've got an old slant cab from 67. It's got the paper voice coil 20w G12M's in it. The previous owners were Brian May of Queen, George Lynch from Dokken, and some band named "Nazgul".
It's survived intact for 45 years and it's rated at 80w. It's not impossible to run speakers for a long time if you're careful.
If you're not, then you'll be smelling burnt wire and cooked glue...not very pleasant, I can tell you that much.
Hope that answers everyone's questions. Been up since 4:42 am working so I'm hitting the shower, grabbing a cocktail and collapsing on the couch!
Have a good night!
Jim