It's not really a life/death issue. It's very true that a rifle will drop off in accuracy as it wears and that is the primary indicator that wear is taking place.
There are some ways to breathe new life into the rifle. If the chamber throat is worn, match shooters have the barrel "set back"...screwed in farther into the receiver. That makes the chamber short so they rechamber with a chamber reamer. It's not a real easy fix and requires some expert machining but it's routine maintenance for match shooters.
The muzzle is another issue. Wear occurs there from rounds fired and may also occur from improper use of steel or segmented cleaning rods. The muzzle crown may also be damaged by other mechanical means. The muzzle end can be cut then recrowned and it gives the barrel some more life.
Eventually, you rebarrel.
Hope that helps.