I know how to spell Buddhism.
Probably not any more than you do. I understand the very basics of Buddha's teachings, I understand that there is significant variety in Buddhism, and all the crap everyone knows. Stekim's the resident Buddhist.
I'm more interested in the anthropology of religion than the content of any given religion.
Studying religion on broad terms, you figure out that religion is a lot like language: The words are only a small part of the whole... but there's always the @$$hole that wants to correct every little word to its 19th century correctness. It is also mostly symbols to communicate something and it changes fast and within very few generations in isolation you get a new one.

The way I see it the reasons Buddhism spread parellel the reasons Christianity spread, while Hinduism and Judaism both represent older religions that survived the new ones and adapted slightly to keep up with the times.
Looking at Islam's founding, you see one of the things going on here. Muhammed was shocked that the Jews and Christians didn't jump on board with Islam. Instead he converted a lot of pagans.
Similarly Christianity replaced pagan religions in the Roman empire, but was only a small cult movement within Judaism. And Buddhism spread throughout Asia displacing their pagan religions or integrating with them, while becoming virtually nonexistent in India where it was born.
The reason? These religions had plenty to offer the pagans, but little to offer their parent religions.

That's why regardless of whether Christianity was born out of Judaism, we'd still be very likely to see new religions form as regional super-religions rather than continuation of pagan religions. Pagan religions work well for people before science progresses when it seems that most of life is outside of human hands. Buddhism, Christianity, and Islam do more to place the world into human hands by making human action more important. They use the moralistic basis from the evolved forms of Judaism and Hinduism, something lacking from the pagan religions.
Over time, you'll see. Religions that don't keep up with the progress of a civilization or offer its people hope die off. Either that or they adapt. And failing that, someone has a revelation and a new religion or sect pops up.

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"Man lives in the sunlit world of that which he believes to be reality. But unseen by most is an underworld, a place that is just as real... but not as brightly lit... A DARK SIDE!"
-opening from Tales From the Darkside