That's the very reason they can be seen as scams/jokes though. Because the religions we hold with great reverence today had to have been recent once upon a time. From what I know Falun Gong isn't too different morally from how conservative Christianity and Islam observe it so I can respect that. But Scientology and their E-meters is just baloney so that obviously I cannot respect. Wicca is pretty legit because it seems to me a revival of pagan roots, and that's been around longer than any of the monotheistic gods that damned magical rituals as superstition and witchcraft. I can see why Wicca isn't well respected, because Christianity essentially destroyed (or tried their best) European pagan religions in its meteoric rise to become the dominant faith of Europeans. Coming from my cultural heritage, Wicca in terms of its rituals doesn't seem fundamentally different from Chinese folk religion as well as its pantheism so it seems more or less "normal" to me.
I don't think there's a line, because if I was a Christian, born in Canada, I most likely wouldn't be able to relate to Wicca the same way I can presently. It all depends on what's popular or not and whether it adheres to our current paradigms - so a new religion will be respected if it can piggyback on the Big Five, and if it can't do that it still might be able to if it doesn't run counter to a secular, scientific worldview like Scientology does. Actually now that I write that down, that's probably the big reason why new religions can't be take seriously - because they originate in a secular, scientific paradigm where there's no mystical reverence to back them up. The Big Five religions are basically legacies of the past, and even they will become irrelevant one day if traditions can't keep up.