Typically, the end of the season is a time for mass exodus. Nobody ever leaves our gym to go to another (we're the only game in town), but girls leave for a lot of reasons.
The last USAG stats I saw showed that in March 2007, there were 20,000+ level 4 girls. There were just under 9000 level 6s. There were around 1600 level 10s and only 97 elites.
You're probably going to see some fairly significant attrition at the end of every season. There will be lots of different reasons for leaving -- time, money, puberty, injury or chronic pain, or just loss of passion for the sport (seems unfathomable when you've got an 8 or 9 year old that's totally into it, but it happens).
At our own gym, Dd1 and her teammate were the last athletes remaining from their level 4 squad. They were both still competing while in high school, so they really did beat the odds.
Once dd2 leaves, there will be just one girl remaining from her level 4 squad.