My guess would be some commercial aircraft somewhere

Ditto. Commercial aircraft only generate money when their landing
gear is retracted. So there is a large incentive to keep them flying.

On the other hand, most military aircraft actually get relatively
little flying time, except in times of war. This is why the KC-135
tanker fleet is scheduled to fly for at least another 10 years (over
50 years total) and why the B-52 will also be flying into the
forseeable future.

Martin