Heat from radioactive decay and primordial heat are each thought to be about half Earth's internal energy budget - tidal forces from the Moon, even though they're strong enough the change the elevation of the surface by up to half a meter, do not factor in at a significant ratio as far as I know. Of course I'm
only talking about the Earth-Moon system and the claim that without the Moon we would not have any atmosphere or magnetosphere.
> moons of a large gas planet could also harbor formation of life
Indeed, and the fact that there are at least two ways to convey energy to an ecosystem should increase the prevalence of life in general. Moons are an excellent spot for life, in some ways maybe even better than planets. They are more numerous, they can be cozy even outside of the Goldilocks zone, to some degree they can be protected by the magnetic field of their gas giant, they are never tidally locked to the star, and they might have a better chance for volatile chemicals on the surface than your average rocky planet.
Since the thread was originally about the Fermi paradox though, one has to wonder about the likelihood for bearing technological civilizations. How fruitful are ocean worlds for bringing forth tool users? Intelligence may in fact be common, but how conducive is this environment to science and engineering? We don't know.
> though they'd have to be rather closer to their parent star than the Solar System's giants are
If they're experiencing enough tidal stress, such as Europa is, they don't have to be close to the star at all.
