Metal gets fatigued over time when under constant changing stress. At some point it isn't safe to fly in old planes no matter how well maintained.
True..also, these planes do lots of short hops...many take-offs and landings..where stress/load is the greatest..Remember the Hawaiian Airlines plane that lost the top of the fuselage because of metal fatigue....That airlien does short interisland hops, so that airframe had nearly three times the number of evolutions as most other planes of the same hours of flight. It ( metal fatigue) was the contributing cause of the incident.
Not to mention when that metal spends its life around seawater.