Forget all the minor concerns about nuclear proliferation and cost. RTGs just have a very small niche of useful applications RTGs are basically just a thermoco

Forget all the minor concerns about nuclear proliferation and cost. RTGs just have a very small niche of useful applications RTGs are basically just a thermocouple stuck to a chunk of radioactive material, which gets hot. They are dramatically less efficient than a nuclear power plant, so it really doesn't make sense to use them in situations where a nuclear power plant is an option. That limits them to relatively small things (because your submarine or ship or whatever can have a proper power plant), that can't be connected to a power grid (because you could just have your power plant elsewhere), even periodically (because you could just use batteries), that can't harness more easily available sources of power like the sun or wind, and can carry the shielding necessary to make them safe. Ultimately, that narrows down their applicability greatly.