Like flying to the moon, colonising Mars isn't a 'bad' idea. It won't harm anyone, most of humanity will keep on doing what it's doing. It doesn't take away important resources from environmentalism or other pressing causes, that's a zero sum fallacy. Musk's point is to push forward relentlessly to a future which incrementally improves humanity's chance of survival. Even if we stop messing up earth, an asteroid or a north korean engineered super flu could still end us while we watch solar powered TV and eat kale chips.