This was basically with an assumption that the salesman does not learn additionally with every passing year of experience. This is where the paradox lies. Those who learn with every passing year of experience, move on to the next level and no more remain salesmen. Probably the most critical factor is keeping your mind open to the experience you have every moment. That is when you make the job more interesting to yourself as well to others.
Many athletes experience a certain exhaustion after a few laps.
It is never the given role or position that itself is important or interesting. It is the person who is holding that position who makes it more meaningful, interesting, contributing and full of learning. They say, travelling makes people wiser. How you travel that is more important than how much you travel. Just as how you live your life is far more important than how long you live.