What is the actual goal of environmentalism on earth?
If it is to preserve diverse species, then it absolutely makes sense to colonize other planets and spread the wide variety of plants, animals, and other organisms. The earth will eventually be burned up by the sun.
Let's keep a sense of perspective here. What the sun's going to be doing a billion or so years from now seems like a rather remote concern compared to entire countries being underwater in a few decades.
If interstellar space travel is ever practically attainable - and, FWIW, that is still a controversial subject - then humans absolutely will figure it out. Or rather, they will just so long as they manage to extend both their existential and economic trajectories far enough for it to matter. Ain't nobody gonna be able to develop that kind of technology from Mars; they'll be way too busy on more prosaic tasks, like repeatedly checking their domes for leaks.
If it is to preserve diverse species, then it absolutely makes sense to colonize other planets and spread the wide variety of plants, animals, and other organisms. The earth will eventually be burned up by the sun.