Like desalinization and hydrogen power that you mentioned above, hydroponics is also energy intensive. In this case it's energy-intensive because you have to isolate nutrients and pump them into water to then feed to the plants. Hydroponics is cool, for now, especially where space is cramped like in cities, but much beyond that and into the future, there's nothing particularly promising about it as a way to sustain a huge population. You have you look at how many calories of energy it takes to create a calorie of energy (aka the EROEI -- energy returned on energy invested). When that ratio is very close to one, much less at 1 or below one, its promise to sustain a huge population in the future is zero.I can see hydroponics get bigger as a way to produce food indoors in any climate.
We'll much sooner run out of fuel, including the massive amounts of fuel we'd need to be able to dig out subterranean centers.I think longer term subterranean population centers (underground cities) could become popular if we start running out of land.