The ultimate energy source, for which we all hope, is to master controlled fusion. We're not there yet. So we look to other sources to fill the need as fossil fuel supply dwindles. Together, solar, wind and geothermal may be able to bridge the gap. If, as some suggest, fusion will forever be illusive, then I'm afraid we're already screwed.
As to your question, IIRC, at current consumption rates, we would barely make a dent into the stored heat inside the Earth; however, you are correct, if we continue to grow consumption and suck heat out indefinitely, it will eventually make a difference, but that is hopefully far enough out into the future that it permits us to perfect fusion.