You know whenever I hear someone describe TOS as "the ship and crew were completely on their own without a Starfleet Command to call for backup" I honestly have to wonder if that person or persons has ever watched TOS, because that was not really the case.
In fact they frequently running into other Starfleet vessels and stations. And if they weren't doing that they were visiting federation member planets and colonies, or had dealings with federation officials.
Frequently, yes, but not constantly. There were other episodes that were predicated on the ship being isolated, having to wait weeks even for a response from Starfleet Command, so that Kirk had to be the one making decisions that could make the difference between war and peace. "Balance of Terror" is a prime example. The point isn't that TOS was always like that -- the point is that it could be
like that, could tell that kind of story in a way that TNG rarely could.