People do not think a baby boom is as worrisome as immigration. Nor intranational migration.
Yglesias:
When people hear about a town that's attracting many new residents, they
say it's "booming" not that the newcomers are poaching a fixed supply
of jobs. Nobody in Texas seems to have proposed trying to close the
state to migrants from the Northeast and Midwest; rather, they see the
state's attraction to migrants as one of its strengths. The
"foreign-ness" of newcomers from other countries distracts people from
fundamental dynamics that they understand in other contexts.