it takes a lot of space and makes very little energy

That's not true of renewable energy, so your analogy has already fallen apart. It's an untrue stereotype concocted by people trying to hold on to their fossil fuel profits.

if the energy throughput generated by renewables is less than our consumption

It isn't.

then we still need a different source.

And thus we don't.

And what will we do when all the space is used to and we still need more energy?

Well for one thing, most sources of renewable energy can be built places where a huge nuclear power plant can't, such as in the ocean, lakes, on hills or even mountains. In deserts. On a slight incline. Somewhere without a cold stream.

If anything, renewable energy is much MORE space efficient since it doesn't need a huge flat area and the aforementioned rarer and rarer stream.