Good sci fi is written so that it can be plausible; it's depicting the author's vision of what may happen in the future; so if you have a human who shoots laser beams out of his eyes and farts ****-fueled nukes, you better have a damn good reason for it.
You can't write sci-fi and just go "Oh, well it's sci fi, so I can make ANYTHING happen!"; even Star Trek tried to quantify what it did by using a bunch of complex-sounding techonbabble.
Plausible/believable is important, yes, but is an elastic term that is defined by the requirements of the storyline and setting (just look at fantasy books). In the case of BSG, specifically, what is plausible within the universe can go in a multitude of ways; one way (not that this is what I'd say is the most likely or a way I'd expect to be taken) would be if the prophecies & history referred to could be taken literally, so you'd have beings who were close to being or actual gods living in Kobol alongside humans; in which case a lot of stuff would become more plausible. Likewise, if it was found to be not literally true, certain things that might happen based on the prophecy, etc, would become more implausible. With, for example, the date of BSG being unknown, it raises a lot of issues as to what is the most plausible connection to Earth, particularly Earth as we know it, is.
If the writers find some better way to tell the story of going to Earth than making it the origin of humanity and hence it's (our?) journey to Kobol in the first place, then I'll judge that based on how well that part of the story is told and written. Yes, it will likely be a more difficult way to go based on BSGs fairly 'solid' ground; that's why I don't think that route would be taken, and that's why my personal preferred option would still be the Earth-as-origin storyline. But until we are given further information about Earth and the history of humanity as it is in BSG, then it's impossible to really judge which options have been closed or opened.
Assuming they ever reach Earth to answer the question, of course.