The danger I see in a lot of arguments for, essentially, abandoning earth is that they seem to be giving up on solving our self-made problems and opting for escape, like the humans in Wall-E. I see three problems here:
1. It is basically giving up on the vast majority of humanity, not to mention the rest of nature.
2. It's an excuse.
3. It relies on magical thinking. If we can't solve these problems, how are we going to solve the massively more complex ones presented by space travel and colonization? Conversely, if we can solve those problems, then surely we can solve these ones first.
Edit: Like, we already know how to fix climate change. There are plenty of perfectly viable plans that involve far less expense than space colonization, for which we don't even have the beginning of a plan. And yet we lack the will to deal with our self-created issue - a substantial number of people refuse to even accept that it is a problem. But space travel will fix things?