Its not really speculation. The planet recovered from much worse scenarios. It was at one point just a rock of hot magma stuff. Humans will die very quickly if things get nasty but we are just very sensitive creatures. As soon as we are gone, things will continue to be weird for a couple thousand years or maybe much more, but eventually it will just stabilize again like with any other extreme period in the planets history.

The conditions will just be different in ways that are incompatible with us, but short life cycle fast evolving creatures will adapt no matter what. From a nature perspective this is just a bit of a strategy shift, because as long as there is some sort of atmosphere (which there will be without a doubt) things will just go on. Just without us.