Why go through all this trouble that they did in the movie? I think there were 11 possible planets similar to Earth that could be colonized from today's findings. By 3128 (the 31st century), the number should be in the hundreds.
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They did find some places to go, like drifter colonies and the like or joining up with aliens. But I think one of the main issues would have to do more with the Human Race's power base and a large percentage of their population being destroyed. With it being shown that a great percentage of the population didn't get off. (with even some of the ships that managed to escape being taken down from debris of Earth's destruction) And after a situation like that, the people may have disagreements over what to do. Like whether or not try to laboriously start from scratch somewhere, or just try joining up with some aliens who already have technology and the like that the humans already know as refugees. (Such as Tek, who resided on Earth before its destruction for instance. So they did apparently have strong relations with other alien races.)
Also (and probably more importantly) given that the Drej destroyed the Earth as a form of ethnic (or species in this case) cleansing to destroy the humans for fear of their potential (by getting rid of their power base and killing off many if not most of their people, hoping the rest of the species would fade away slowly but surely while dispersed into space. At the very least that could stop them from ever becoming as powerful as they used to and have the potential to be.), it is very possible many would fear trying to make a planet of their own on an official/large-scale level lest the Drej attack again to continue what they started. (Like with the hunt for the Titan, they're shown to actively try and stop the human race from doing anything like that. So who knows, maybe some people could have actually tried or discussed it.) The ending implying that they will now all have a home, and it is with the Drej finally eliminated and no longer a threat. I'll probably agree that they should have explored these notions more, but I don't think it's completely implausible/unexplainable. (within the bounds of its own sci-fi rules that is) Though that is just in my opinion.
The human's resources must've been spread real thin after the Drej destroyed Earth. Colonizing and terraforming a new planet takes a concentrated effort. Even if they tried, I'm sure the Drej would just get a hint of it and just attack and blow it up like Akima said about the Titan.
shareFine enough, the human race is too dispersed with far little resources in between, but that's diverseness seems to me like an advantage to me. If you're spread out, then the probability of finding a planet similar to Earth grows. If they don't have the time or resources to terraform a planet, then find one similar to Earth instead. We've seen that faster-than-light travel is possible and maps detailing vast spaces of space are used, there should be little excuse.
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I think there were 11 possible planets similar to Earth that could be colonized from today's findings.