It's something of a natural reaction for us to take a step back and a deep breath when we finally reach the pinnacle of our journey. When the end of the school year arrives, we usually don't spend the next day doing homework! And in theory, not only is there really nothing wrong with this, it can be significantly healthy for us to take a step back and relax.
But like everything else in life, when we stop using a skill, a muscle, or a part of our brain for a significant period of time, we can forget how to use it. It's similar to how we might forget how to get somewhere if we haven't been there in a while. And officials can't let themselves do that.
The real point, though, is not so much about reaching the end of the physical year, but rather the top of the profession. When the baseball season ends, umpires don't look for another opportunity to strap on the gear and work the plate. They cherish the time they have at home, relaxing, spending time with their families, etc. But the good officials realize that even if they're the oldest, longest tenured officials with the most important assignments, they have to maintain that level and continue to get better, rather than "throw their weight around."
The job of the official is to start out perfect, then get better from there. Think about that for a second...that sounds like something Yogi Berra would say! But it's true. No matter your profession, remember that complacency at the top opens up an avenue for someone beneath you to usurp your glory.