it is actually a snowballing problem. Imagine if you need 4 working adults to support 1 oldie. So, you open the gates and allow for large influx to satisfy this need. In time to come these 4 workers today will need 4 x 3 = 12 workers to support down the line, 12 will then need 12 x 3 = 36...with people living longer, you can expect this number to keep growing.
Singapore land mass won't keep growing, but the large population (3.5 - 4m, I discounted those transient workers) will mean larger % of older folks that require larger working population to support...and the numbers just grow.
which is why the government says keep working longer so that an oldie dun need so many working adults to support. However, if you look around, there is generally no support for older workers and if they are gainfully employed, they are earning pittance, except of course you know who, whose pay check is guaranteed for life.