im still skeptical about what you said.
imagine you have a completely sealed container. you take a cup of starsan, put it inside, shake it up and coat the entire inside surface with starsan. then you let it sit there and the starsan pool at the bottom. i dont know where the bacteria would be able to get in from. i suppose you could argue that it is suspended in the air inside, but as soon as it touched the inside surface it would be killed because of the starsan residue. even if, by chance, the residue wasnt enough to kill that single bacterium, it would be killed the next time you shook the container and re-wet the surface.
assuming that the starsan didnt go bad while it was in there, i dont get the difference between putting the starsan in the carboy now, versus putting it in later right before you put beer in it. its the same exact sanitizer, regardless of time.
i would even argue that it is better to put it in sooner, as bacteria grow in numbers exponentially. if you do not sanitize the carboy before you store it, you are storing it with a drasticly higher bacterial concentration to start with, which, would have an even better chance to grow.
so for example, if you stored it unsanitized for 10 days, starting with a bacterial count of (pulling number out of thin air for example) 5 million. say they gain 5% per day. that is 81 million bacteria after 10 days. if starsan kills 99.9%, you are left with 814,000 bacteria after you sanitize it before use.
if you start with 5 million bacteria (unsanitized) at day 1 and put sanitizer in there from the begining, you end up starting with 500,000 bacteria. say they grow the same 5% per day, that is 814,000 after 10 days. you then sanitize it to kill 99.9%, leaving only 81,000 bacteria. that is 10 times less bacterial load.
that is all assuming: bacteria can grow at the same rate in a soloution of starsan as they would naturally, which i would question, and that the starsan lost its effectiveness after only a short amount of time, which we know it doesnt. im sure its mostly moot, though, and i am making it way more complicated then necessary. people have been making beer with great results even before starsan was invented ( i know, right!).
thats how it works in my head anyway... its a crazy place in here.