I am having a brain lapse. When did Enterprise return to Earth during the course of TOS? I mean real, 23rd century Earth. Not Guardian of Forever Earth or black star slingshot 1967 Earth but good ol' 2265 Starfleet Earth?
As for taking extra supplies... I climb mountains. If I go on a three day mountain climb I take three days provisions plus two days light rations in case of emergency plus the gear to help find and process consumibles if the emergency goes longer. I think the very idea of "beyond the frontier" and "unknown" demands preparation for catastrophic failure. Roddenberry must have agreed as he said twelve out of thirteen ships experienced such failure or something similar.
Look, within the context of TOS, TAS and TMP it is a five year mission into the unknown that nobody else pulled off. Add in the other series and movies and it might muddy the picture but hey, that is always the case. That is, after all, what this thread really started out being about, right? A fan whose dad fleshed out the TOS universe for her and her friends whose work was universally accepted by fans the world over and formed, literally, a cornerstone of fandom to the point it came to piss off DC Fontana, Matt Jefferies and Roddenberry himself, and continued to be accepted as such until strenuously and deliberately undone by the very studio (and its employees) that had so benefitted from its act of resuccitation. And the coda - resuccitated and reborn as a dumbed down product (the TOS movies) for an expanded, unquestioning fan base that made Gulf and Western stockholders a profit but hardly did much to make that TOS world "real". Real in the way Franz Joseph had done and for which TOS fans had seemed to hunger. But hey, a hungry fan doesn't demand steak when tossed hamburger.