B.C.
Neighborhoods are not static.
They change all the time.
Families have babies and move up to bigger houses.
Kids graduate high school, leave for college and then families move out looking to down size.
Job transfers happen and whole families pick up and move.
In the neighborhood I grew up in it varied quite a bit between when we moved in and when I left after college.
Kids moved in and out all the time.
If the schools are good and I like my house/yard (and there are no feuds going on with crazy neighbors) then there's no reason to leave.