The potential lack of vulcanic and geological activity now is a big problem. For the first 700 millions years life on Earth depended on chemosynthesis and that's where the energy for that comes from. Presumably there could have been more active geology far in the past, though, possibly implying life that died out.
Isn't surface temperature a good proxy value of the amount of energy inside the system?
Well not necessarily. The Martian equator can get as hot as 18C. It doesn't retain the heat (due to no atmosphere) and it gets cold like even a few centimeters off of the surface.
Wow, I learned something new and can now think about the world more closely to how it really is. Thank you.