Unfortunately, you, like most sinkies DON"T EFFING GET IT. The PAP plan for it never to reach 99 years. The 99 years is a smoke screen to lull the "buyer" into paying for the flat and thinking its a long way to go. In fact, no HDB flat has exceeded 30 years. The flats themselves are not designed to last 99 years. Look at the construction and engineering aspects of that.
The PAP plan to make you fork out the money somewhere around the 25-30 year mark. They will come and tear down your whole estate at that time, and offer to relocate you to a new estate. But of course, you must pay more for your new place, and it will re-start the 99 year lease clock again. The HDB has no interest in seeing you reach the 99 years. Why? They can sell you 2 flats within that time frame. Just look at some of the original estates like Queenstown and you will see what I mean.