The existence of extraterrestrial life does not necessarily mean that the ancient aliens hypothesis is true. That said, I'm uncertain as to how it would contradict all religions. Perhaps some of the "lesser-established" sects so to speak like the millions of Evangelical offshoot denominations and maybe some Islamic Wahhabi sects (the Evangelical fundies of the Islamic world) would collapse, but the main established bodies of the big religions like the Roman Catholic and Eastern Orthodox Churches for Christendom and whatever Islam, Judaism, and Hinduism's equivalents are would also probably survive. I can't speak for the other religions because I do not fully understand their doctrines and teachings and therefore cannot give an informed answer as to whether their theology is compatible with extraterrestrial life, but I'm pretty sure that even if it is/was not, they could still adapt it to the reality. Religions have been doing this all the time--rethinking and developing their worldview with the new obstacles that they encounter in the world. Only the fundamentalists who cannot concede any change or development will die out. So you can probably count on Evangelicals and Islamic terrorists to decrease in number. I know from my own Eastern Orthodox religion that extraterrestrials wouldn't really contradict anything. We tend to see our religion and the effects of it as a cosmic event--in other words, that the events we believe Christ accomplished affected not only the human race but the entire cosmic reality of the universe as a whole. Therefore, it probably wouldn't be too hard to reconcile this with extraterrestrials.