This! Imagine how much food could be produced just by putting a greenhouse on top of every appartment building. At that point vertical systems of agriculture will become commonplace.
Necessity drives change. When food gets too scarce or prices get too high, we will implement some of the above practices. Before we even get to that point, it may even make sense to reclaim developed areas and turn it back into arable land.
Also, there are bound to be at least one major war, a few new diseases and the occasional natural disaster over the next little while.