To me, if mars had life in its past and no longer does then we could consider it one huge grave. Lost in the terraforming debate I read is that any atmosphere we managed to kick up would be temporary. It might take a long time to leak off, but they’re not discussing any means to reverse the trend. As I understand, Mars doesn’t have an active core and thus magnetic field holding its atmosphere in place. There’s no way we’re going to fire up a core…

Turning choice, large asteroids into liveable habitats seems an infinitely more productive task.