Radioactive decay is a component of geothermal heat, but it is not all of it, quite a large chunk of it is simply residual heat from time of the formation of the planet.
"The flow of heat from Earth's interior to the surface is estimated at 47 terawatts (TW) and comes from two main sources in roughly equal amounts: the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust, and the primordial heat left over from the formation of the Earth."
Most of the heat flow from the thicker continental crust is attributed to internal radiogenic sources, in contrast the thinner oceanic crust has only 2% internal radiogenic heat.
In the Swiss alps most of the heat is coming from local radioactive decay.
