It may look predictable but I feel it is far from it.
To many variables come into play.
Take Avon (IN) for instance. Would you believe roughly 35% of their band this year was either a freshman or a first time marcher? That is about 90 kids! Makes you wonder what the instructors think everytime a new season starts. I can't imagine having to do that every year and next years class will be slightly bigger
When Avon started winning consistently, I feel that is when the students picked themselves up and decided that this is something they want. That is also when they realized that it is harder to stay on top than to get to the top.
In some aspects having a dynasty is good. It marks a point in time where a change to a particular subject has occured, and people can look back on it, study it, learn it, understand it and what impact it had on the profession.
Every school with a competitive marching band will leave their mark on this sport whether or not they make it to the top.
Some will leave a bigger mark but not out of spite, greed or selfishness but out of the joy of doing something they love. What a way to depart!