Leasehold is better for the nation. After 70 years, the buildings are probably too old to maintain, better to tear down and rebuild. If freehold, you need to get agreement through voting, a complex and time consuming process. For leasehold, government has a choice to ask for a fee to continue the lease for x number of years or tear down and rebuild, very flexible depending on the desirability of the neighborhood. Leasehold ensure rejuvenation.
Btw, 70 years is about lifespan of the purchasers, assuming most of them bought at the age of 25-35.