The HDB flat with a 99 year old leasehold is a declining asset, because it appears that the government and HDB inlcuded the cost of land and the cost to build it, in its "price" to the citizens. But at the end of the 99 years, it is worth nothing, and the land belongs to the state.
Therefore, during the boom years, when there is significant demand from citizens and PR's, the artificial price may increase, but after 30 to 40 years, it will not appreciate in value as it will decline.
If the government does not have any intention to make a profit from the citizens hard earned money and CPF, it should sell the HDB flats at cost plus a margin for administrative costs, but not to include the cost of the land.