The economic effect of positive net immigration to Singapore is negative relative to older people working longer. When the population base remains stable and retirees extend working life, we have a higher GDP per capital.
If we continue to bring in more immigrations without an end, there will be more stresses on our infrastructure. Comparatively, except for healthcare infrastructure, old people demands lesser for road usage, transport services and natural resources.
Intuitively, it makes more sense to retrain our aged population and to encourage them to work longer than to expand our population without end. This is perhaps why Japan rather deal with an aging population than tolerant aggressive immigration.
(I understand the need for a critical mass of population. Perhaps 6.9m or 10m is the defined figure, but there must be an eventual limit to our planning parameters)