You might want to compare Singapore with a city like NYC or London, not with a territorial state. It's pretty normal around the world for cities to be replenished mainly by people moving in.
(Of course, to be fair you then also need to compare GDP per capita against other cities. And they usually do a lot better than territorial countries that include a lot of hinterland.)
> A low TFR is indisputably the slow death of a society.
No this is a non-sequitur.
Low fertility only means that the affected society grows smaller. This is only equivalent to "slow death" when it persists for centuries until the society is actually dead.
Whereas a society replaced by its own children maintains continuity of community, nation, or broad grouping of people having common traditions, institutions, and collective activities and interests
You might want to compare Singapore with a city like NYC or London, not with a territorial state. It's pretty normal around the world for cities to be replenished mainly by people moving in.
(Of course, to be fair you then also need to compare GDP per capita against other cities. And they usually do a lot better than territorial countries that include a lot of hinterland.)