Your example of a typical condo retired couple is not typical. Many opted for a more modest lifestyle. They no longer drive a car because it is too costly and they don't need it. They either live next to an MRT station or their condo has a shuttle bus service to bring them to the nearest MRT station if they need to travel. They also no longer need a maid as it is not difficult to maintain a two retired persons household. They may also not spend too much on eating out at restaurants.

A more realistic typical condo retired couple has these montly expenses:

Food, and food items and occasional dining out, $1000
Utilities, $250
Medical and health insurance, $700
Holiday, once a year (pro rated), $200
Condo fees (condo assumed to be paid off), $350
Misc, $500
Total, $3000 pm or $36k pa.

So, if they are able to generate a passive income of $36k pa, then they are ok. If they are lucky, their kids will give them monthly allowance of $1000 pm to each of them (assuming 2 kids, each kid give $500 to father and $500 to mother - could be more if they are luckier). They can save these allowances.

To generate $36k pa of passive income is not that difficult. You can get from the CPF Life, your blue chip investments, etc.