Its a great idea for hdb to sell at 50 years lease and should the owners sell 5 years later and the buyers are able to buy at 99 years lease. Assuming if the price of the flat goes up to 800K after 5 years and if they bought at 400k. HDB is entitled to 49/99 ratio (base on the lease) of the 400K appreciation. Win-Win for public and government.