It’s happened before.
Writing a century and a half before the birth of Christ, the Greek historian Polybius observed “nowadays all over Greece such a diminution in natality and in general manner such depopulation that the towns are deserted and the fields lie fallow. Although this country has not been ravaged by wars or epidemics, the cause of the harm is evident: by avarice or cowardice the people, if they marry, will not bring up the children they ought to have. At most they bring up one or two. It is in this way that the scourge before it is noticed is rapidly developed.”
He concluded by urging his fellow Greeks to return to their historic love of family and children. “The remedy is in ourselves,” he wrote. “We have but to change our morals.” His advice, unfortunately, went largely unheeded.
The demographic winter of the Greek city-states led to economic stagnation and military weakness, which in turn invited invasion and conquest. After a century of increasing dominance in the Eastern Mediterranean, Rome finally annexed the Greek city-states in 146 B.C.