Declining birth rates are a standard feature of modernization across cultures and it's only a problem because cultures like the US get stuck in a rut of trying to preserve premodern cultural modes that are no longer useful, like the belief that society needs millions of tedious, low paid factory jobs to be successful or that women must be forced to have children to preserve population levels (and racial caste systems).
Immigration is one of the most straightforward solutions to local population decline but the racists lose their shit over it every time.
In the long term, a level of social reorganization is necessary, and yeah, it probably won't look like either small town America or modern urban sprawl, which is okay if you're not deathly afraid of change.
@gwynnion I'm rather hoping it looks like modern urban cores, because *fuck* suburban hell, but also *fuck*, with the fire of a thousand exploding suns, small towns.