The default is - the land goes back to the government and you get $0.
Some areas are being selected for lease extensions and other en bloc development (but that happens well before the lease is up).
But the government has come out and said “there will be few leases where that happens”.
that being said, if it weren't, the concept of a 99-year lease wouldn't have any teeth.
indeed, your point is a central pillar of the land development policy in singapore -- the idea that things evolve and the government needs to make decisions for the 'greater good'. the same concept exists in the usa (imminent domain; a completely different implementation but still one that strips assumptive rights that a property owner has).