I definitely agree that gravity, some sort of ecosystem, probably some kind of atmosphere, and a source of heat are needed in any case. The idea of "stone age"

I definitely agree that gravity, some sort of ecosystem, probably some kind of atmosphere, and a source of heat are needed in any case. The idea of "stone age" tech being enough to survive implies an ecosystem that maintains itself. A rogue planet could have heat and water if it was reasonably large or part of a multiple body system with tides. C.f. Enceladus and Io. Then there's Titan.