Stress cycles and aluminium are your limitations. The A-10 flies a pretty dynamic mission. Now B52s, which mostly fly high and straight, are a different story (takeoff-landing cycles affect wings). The newest B52s were build in the early 1960s. They'll be flying through 2040, an 80 year lifespan.
Ultimately it's wing fatigue that takes them down.
The key words in the post I was replying to being "indefinitely" and "proper maintenance". Yes, if you rebuild your Ship of Theseus, it may well last forever. But that raises other questions which have been debated for some time.
Aluminium has specific concerns in that when it fails it has a strong tendency to do so catastrophically and with no warning, unless you've been very closely examinging for developing stress fractures. I've had personal experience with this, and there's a rather well-known video sequence of a fire-suppression air tanker undergoing an unscheduled dewinging above California a few years back.
Cumulative entropic stress throughout the airframe does ultimately raise concerns over predictability of structural integrity. The more dynamic your flight patterns, the greater those stresses are, and more unpredictable.
Ultimately it's wing fatigue that takes them down.