We can accept from known Earth biology the successful concept of evolved symbiotic relationships... and we know from "The First Ones" that the Unas were already aware that the Goa'uld would try to blend if they could (Chaka seemed to anticipate the snake's intent when it flew from the water at Daniel), and took measures to stop it (he was wearing the necklace thing). And indeed the "raw" snakes did blend with members of the search party! So, we can surmise that on this "primitive" planet we have two species which "enjoy" an ongoing, and presumably long-term, if slightly one-sided symbiotic relationship.
That could go on for hundreds of years - snakes swim, "jump" at the nearest passing host (either uneducated, foolish, or careless) and formed a symbiotic pairing. And we try not to think about the icky matter of procreation.
As you say, the big difference would then come if and when an outsider arrived, perhaps via the Stargate, or as an Ancient (or other species sub-contracted to install the Stargates
) during Stargate installation or commissioning. And happened to be too close to the water, all unawares.
The question then comes at what point the newly blended alien host "helped" some of his own species to also get new hosts.
"Thor's Hammer" implies that either the blended Goa'uld/Unas lived for a VERY long time without a sarcophagus, and/or the first blending occured only a (relatively) short time before some other species arrived on the planet and allowed the new hosts to escape off-world.