A small story about a hard-turning key
The customer had already learned the trick. Pull the door, lift the handle, turn the key.
The trick worked until it did not. The lock received the blame because the key was in it, but the door had been pressing the bolt against the strike for months. A new cylinder alone would have been a neat-looking answer to the wrong problem.
This is the kind of experience we mean when we talk about 20-plus years. Not a claim that every job is difficult. It is the habit of watching how the door behaves, asking when the symptom appears, and looking for the least disruptive reasonable fix.
Sometimes that means an adjustment or repair. Sometimes the hardware is worn and replacement is sensible. Sometimes the lock is fine and the old keys are the concern, so rekeying fits better. The inspection should lead the recommendation.