Straight lines don't exist.  They are, in fact, theoretical. 

Now you may encounter lines from time to time that seem straight, but if you continue them long enough, they will veer.  The Earth, being round and all, pretty much sees to it that nothing will travel in a straight line, that's of course if gravity can be overcome. 

Consider this, a machine would be considered to be more reliable than humans to produce these straight lines, but even the most precise machinery in the world, namely the space shuttle, spends an estimated 97% of its time correcting it's course ... achieving more of a zig-zag pattern, rather than a straight line.

That being said, "maintaining your fitness level" simply can't happen. 

There's no way to stay in the same spot. 

Age-related changes in your body see to that.  In a sense, just maintaining all of your fitness parameters requires improvement, which makes maintenance a myth.  You either have to improve, however slightly, or encounter a slow, steady decline.  Paper airplanes can glide through the air, appearing to fly, but in reality, borrowing a line from toy story, "That's not flying ... that's falling with style!"

Just for the sake of argument, let's assume that maintaining your current fitness level could actually happen.  It would be such a fine line that it would be nearly impossible to follow exactly.  Chances are that you would ever-so-slightly improve or ever-so-slightly decline.  It may be imperceptible at first, but extend it over the course of a month, or a year, or a decade, and let's see where you are.  Do you really want to be a few years into a slow, steady decline, combined with the momentum that comes along with it, before you realize you're on the wrong track?  Why not err on the side of improved fitness?

Human beings are goal-seeking organisms. 

When we're not growing, we're dying.  We must fight to stay alive, and simply "maintaining" isn't going to cut it.  I try to live my life by the mantra, "Good enough is simply not good enough."

From a physiological standpoint, hormones are the reason for the lack of ability to simply maintain.  There is an anecdote that if you put a frog in a pot of boiling water, it will immediately jump out, but if you put that same frog in room temperature and gradually raise the temperature, the frog will get used to the incremental temperature changes and will remain in the water until it dies.  Gradually and incrementally, a body can get used to just about anything.  Accordingly, the systems of our body become accustomed to physical demands.  In response to repeated demands, muscle fibers increase in size, hormones are either increased or decreased, heart rate beats stronger and slower, lung capacity increases, etc. 

Pretty much all of these changes are due to hormones.  When you exercise at a certain intensity, you release a whole cascade of hormones into your blood stream.  The repeated appearance of these hormones over time will cause your body to adapt to maintain a state of homeostasis (meaning everything wants to stay in balance and harmony).  After a period, the same workout will not elicit these hormones any more, which means that the body will not have to adapt any further. 

Homeostasis has been achieved, but now doing the same exercise routine is almost equivalent to doing no exercise at all, and the decline begins.  In order to avoid this, we have to continually increase the challenge we place on our bodies, so the hormones continue to be stimulated and we stay one step ahead of our bodies.

It has been said that the worst is not the enemy of the best.  The enemy of the best is "just fine." If this sentiment is shared by Voltaire, Dave Ramsey, and Jim Collins ... it's good enough for me!