4% per annum bites really fast, I don't know if people realise that it has the same math as compound interest, it just seems slow if you're looking at units smaller than decades. If we double our use every 30 years like you said, we're also going to start having a sunlight shortage in 300 years. We could start using fusion then, but we shouldn't, because the waste heat would start to cook us, and would be enough to boil off the ocean in roughly another hundred years of growth.

What actually has to happen, is that we pick all the low hanging fruit as far as infrastructure goes, and growth just slows. From a market perspective that probably means returns will get smaller and smaller, making things that were once too expensive to be practical the new best option for investors.

but still, it seems like another “climate-change-like” moment to literally suck up all heat the heat being conducted to the surface of the planet from the interior.

No joke! And, if we wanted, we could go even deeper once the upper layers are cool, since drilling would become easier. I'd guess we'd see less earthquakes, because enough heat removed would shut down plate tectonics entirely, turning Earth geologically into Mars. Deep sea vents might turn off. Probably other bad stuff would happen, but I don't know what exactly.

So we should probably use geothermal energy very wisely.