HDB was incepted to provide affordable rental flats for Singaporeans who couldn't afford to buy landed properties or rent private apartments. It went parallel with CPF to provide for retirement funds for these people. However, somewhere down the line HDB flats for sale began to take over and along the line, rental flats were reduced to very few left. Have Singaporeans' real income increased so much as to justify that mass transition? I don't think so. The money that's sustaining it comes mostly from CPF.
The in-vogue message now is that you can't afford to retire if you don't save enough. But for lower-middle income groups, save from what? Everything's so expensive. If they faithfully complete a 30-year mortgage, they'd end up with a flat to retire in, yes, but not much cash left and no more income if they stop working. PUB, SCC etc. bills still have to be paid. They still need food, other necessities and amenities.
If they sell their flats and get lump sums in the hundreds of thousands, they won't qualify for whatever few rental flats left. They'd be asked to buy another one, in cash, since they have the cash and by retirement age, don't qualify for mortgage anymore.