It's an insurrance carrier call and not an airline call.
It get's really dicy on older equipment where the fleet need would just as soon do without, or, spooked customers refuse to ride on a plane that was so unlucky.
More to the point -
It's the cost-to-repair versus the MARKET VALUE of the repaired aircraft. Since older aircraft, especially less desirable/less efficient models, don't command high prices, they are less likely to be repaired, and thus more likely to be salvaged.
(This is illustrated by the antique aircraft marketplace. A recently-pranged local P-40 will certainly be re-restored to better than new, whereas during WWII it would certainly be scrapped. The difference is the PRESENT MARKET VALUE of the ship.)