Future singapore economic growth, or GDP growth, requires building more infrastructure, like mrt, homes, offices, parks, roads, power stations etc. To justify massive infrastructure building, need more residents to pay for these stuff.
With economic growth, the govt of the day can then accumulate surpluses as well as keeping up with military hardware upgrades. The SWFs will continue to get findings to expand, to help the govt influence regional politics and attract global powers to ally with Singapore.
The above, I think is the basis of wanting ever growing population. With proper planning it can be achieved as there remains large pieces of poorly utilized land, especially the air bases. However, the pap is too stingy to citizens and continue to refuse to properly acknowledge NSmen. Thus the current rejection of population white paper.