As final punishments go, it's not even that bad. He's still alive, he's not alone (the quality of space core as company notwithstanding), and he's probably in orbit around the Earth, so if he has solar panels (and being designed by Aperture Science, he probably does, despite their complete uselessness in his intended operating environment) he doesn't need to worry about running out of power and dying that way. And he probably won't be stuck in space forever anyway.

Given all that, I really can't say the writers circumstance punished him unfairly.