Yeah, this isn't a Marvel comic.
Best case scenario, tardigrades develop to feed on people. Since they are microscopic they get to compete with other of our parasites. Huzzah.
I don't think that's the 'best' case. Dinosaurs evolved from simpler things than tardigrades so if we mess up this planet enough that we have to leave and only the tardigrades survive, our offspring could return to find giant tardigrade type things that eat our not so human children. Or we could time travel forward to that time. The choice of delorean, TARDIS or some kind of planet of the apes time travel, I leave up to you.
