Some historians, like Rodney Stark, have argued that in the next 100 years Mormonism may well become a major world religion with hundreds of members. Stark is hardly an apologist for Mormonism, either.
In general, having "laughably silly" beliefs doesn't necessarily preclude groups from growing and gaining influence. Because Mormons (along with Jehovah's Witnesses, Adventists, radical Pentecostals, etc.) have their own methods for evangelizing aggressively, they can grow much faster than more "establishment" churches which often refrain from doing so.
The Mormon founders themselves envisioned a future where Mormons take the lead in "saving America"
. Many think Glenn Beck's entire approach to politics is based on this idea.
Well I wasn't trying to single out only Mormonism, I think its just as weird as any other religion or Christian denomination.
I did get a laugh at a historian predicting the future. Maybe he's right, but I just find it to be very unlikely. And really historians should stick to storytelling, because that's what history is.