Perhaps calling it a "prevailing" theory was being too generous. However, when it was first introduced it was a rather hot topic. Actually, the presence of a drysap dispenser under an Agaran house was just one piece of evidence. An important piece, to be sure. But I think the most evidence for this theory was certain game lore. In particular, Greenfinger's Notes made mention of drysap. If I recall correctly, some of those mentions were peculiar and highly suggestive of a plot. A lot of the game lore indicates that Greenfinger was working together with Big Ape and Thornwing towards some goal(s), probably sinister in nature. Indeed, it seems that Greenfinger was (accidentally?) responsible for what happened to Earth. Regardless, we'll just have to play the game or stay tuned to the forums to find out. Our theories may no longer be as relevant because Cheerful Giraffe redacted some of the lore and removed some of the codexes.