These have nothing to do with being "established religions." As you said, Mormons shun. Many Muslim groups advocate killing apostates. It might be trivial to get a copy of the Bible, but the vast majority of how e.g. the Catholic Church runs is not drawn from the Bible using the most obvious interpretation, and is not shared with laymen - the Bible is not an IKEA instruction manual.

Also Mormonism is not particularly old. Mainstream evangelical/charismatic Protestantism was formed completely from storefront Pentecostalism in the early part of the 20c, making it barely older than Scientology or even the Nation of Islam. Scientology isn't bad because it's not "established," it's bad because it hurts people.