What would happen to the economies of middle-income countries that is starting to have a low birth rate?

Malaysia is at a point where it's very near to the high-income nation threshold but at the same time, there are signs of a declining population with a birth rate of 1.39 per female. The common reason for the lowering birth rate is higher cost of living and modern attitudes towards having kids.

In addition, it is a country where permanent residency and citizenship is almost impossible to get, and currently most of it's cheap labour is obtained from legal/ mostly illegal immigrants from neighbouring countries.