Fortunately for developers it won't be suddenly. Most land in the city is already occupied: even if the construction and banking industry could rebuild the entire city in two years it takes longer than that to get the current land owners to agree to sell their current house which works just fine for them right now.

Companies only build new offices quickly because they are willing to locate to any new suburb that currently has land. Downtown areas that are already well developed might be more desirable but it takes time to get the land rights. (note they are always re-building something downtown in nearly any city, but it wasn't an overnight thing to decide to do that)