Also, even if there are a lot o unfriendly AIs out there, a friendly one would still vastly improve our fate, whether by fighting off the unfriendly ones, or reaching an agreement with them to mutual benefit, or rescue simulations.
One potential option might be to put us through quantum suicide - upload everyone into static storage, then let them out only in the branches where the UFAI is defeated. Depending on their values and rationality, both AIs could do the same thing in order to each have a universe to themselves without fighting. That could doom other sentients in the UFAI's branch, though, so it might not be the best option even if it would work.
Why do we imagine our actions could have consequences for more than a few million years into the future?
Unless what we believe about evolution is wrong, or UFAI is unlikely, or we are very very lucky, we should assume there are already a large number of unfriendly AIs in the universe, and probably in our galaxy; and that they will assimilate us within a few million years.
Therefore, justifications for harming people on Earth today in the name of protecting the entire universe over all time from UFAI in the future, like this one, should not be done. Our default assumption should be that the offspring of Earth will at best have a short happy life.
ADDED: If you observe, as many have, that Earth has not yet been assimilated, you can draw one of these conclusions:
Surely, for a Bayesian, the more reasonable conclusion is number 2! Conclusion 1 has priors we can estimate numerically. Conclusion 2 has priors we know very little about.
To say, "I am so confident in my beliefs about what a superintelligent AI will do, that I consider it more likely that I live on an astronomically lucky planet, than that those beliefs are wrong", is something I might come up with if asked to draw a caricature of irrationality.