It's been a long while since I studied this stuff, but I believe the heat generated through the natural decay of uranium, with the insulation of earth's rocks, causes differential heating to the interior of the earth, creating convection cycles within the mantle. This drives plate tectonics

Sorry if I'm missing something in your comment, but it seems to imply that the earth's core started out incredibly hot and has been cooling ever since without any additional heat generation, rather, relying on insulation to retain residual heat. It's important to note that ongoing radioactive decay within the earth's interior, together with the insulation you discussed, continues to keep earth's core hot and capable of sustaining plate tectonics.