Historically, we've seen religions replace other (mostly pagan) religions. However, something like that happening in the 21st century seems extremely remote. What aspect of the historical world made it easier for new religions to be born and replace other existing ones?
What about Scientology — a new religion founded in the 20th century.
The founding of the major world religions may look inevitable from present day, but contemporaries of Christ or Mohammad would not have been certain of their permanence and influence through the world. Christianity was a fringe religion for 300 years before Rome adopted it as its state religion as a means to consolidate power. That church is now divided into many denominations who do not recognize the authority of the Pope. Islam burst on the scene in one generation, forever changing the world political landscape. Judaism has outlived more other religions than we can imagine. Who knows what will be true in 300 years, we may all be worshiping alien gods with the Scientologists!