Assuming that Earth was very average is a very rational place to start, but the reasonability of this assumption changes when we look elsewhere for life and fail to find it. Earth being average was once the basis for people assuming that other planets in our solar system might have intelligent civilizations; that has been almost entirely ruled out now (maybe there could be a civilization of space squid under the ice of Europa?) And we've failed to detect any radio signals that indicate life elsewhere in the stars near Earth. There's still a lot of searching left to be done, but each time we look and fail to find anything we should be updating our priors.