Terraforming is pointless anyway. Space-based habitats are a much more efficient way to expand our living space, and most importantly, when living in one, you don't need to live at the bottom of a gravity well.
To paraphrase someone else, when planet-bound civilizations fall to barbarism, people revert to agrarian societies; when space-based habitats fall to barbarism, everybody dies.
The difference being that maintaining a habitat takes a lot of technology that is constantly kept in working order.
When a planet-bound civilization at our level or above falls to barbarism, almost everyone dies. The planet simply cannot maintain billions of people without pretty advanced technology.
In the end, that last % is probably not that significant, specifically because space-based habitats have a very strong advantage over planets -- planets are much rarer. The solar system can sustain many millions of space stations with >1M pop each, with cheap travel between them. I'd estimate that complete system-wide reversal to barbarism is pretty damn rare in that situation.
Modern farming is also a high tech field, most people will die anyway when you revert to agrarian societies. Once you've spread across multiple planets you're no longer at (significant) risk of extinction, so the effect is pretty much the same.
To state that another way, some bad-ass aliens come along and decide to terraform Earth to their liking. It turns out they breathe 100% Helium, so they convert Earth's atmosphere to that.
As humans, we seem to think we're the most important thing getting around. Even your comment suggests killing off humans is much worse than killing of bacteria on Mars.
Back to my story, the bad-ass aliens might very well look at us just like you're looking at the bacteria on Mars.
How does our staying our hand affect the decisions of your aliens? If someone is going to come exterminate Earth then it is imperative that we put our eggs in as many baskets as possible, regardless.
Turning this into an existential question for the humans does not invoke the morality you are looking for.
This form of argument only works on Earth because humans are the most important thing around, and it can be safely assumed that all human-capable things are humans and therefore you can make moral arguments based on the Golden Rule with the safe assumption that we are all working on the same basic desires. As soon as you introduce true aliens into the mix you can no longer make this assumption. You need to establish from some sort of more basic first principle why this is a bad idea.
Might I also add that from my point of view, the question is, which do you prefer, a bare sterile ecosystem barely hanging on and marking time until the sun sterilizes it, or the sort of rich vibrant ecosystem that we can build by terraforming, bringing life abundant to a place nearly sterile. Even on its own terms the snap "Don't touch anything!" is not the life friendly answer you are probably casually assuming. The ecosystem that will exist AD 100,000,000 is quite different depending on whether we terraform or not. (Again, the snap "Don't touch anything!" is itself a terra-centric viewpoint. On Earth you can assume there's a rich ecosystem pretty much where ever on the planet you point, and you can sort of leap to the conclusion that extensive human interference will at least not make it richer. In space, you can't make that assumption.)
In fact this whole thing ends up cutting to the question of exactly how do you compare two possible ecosystems and decide which is better, a question that environmentalists have to date not really had to face but is one of crucial importance even here on Earth, and completely unavoidable in space. If you're up for a challenge, try to rigorously explain why it's important that we leave a starving, barely-functional ecosystem alone for a hundred million years instead of terraforming the place and then in a hundred million years having a rich, varied Martian ecosystem.
> As humans, we seem to think we're the most important thing getting around. Even your comment suggests killing off humans is much worse than killing of bacteria on Mars.
That's because (according to humans) we are the most important thing getting around. No other species has the mental capacity to even consider this point, though every species acts in its own self interest.
To put it another way, do you think beavers care about disenfranchised fish or animals downstream when they build a dam and reproduce? Should they? According to you beavers should seriously consider the ramifications of their behavior. Haughty beavers...
I'm not saying we should completely disregard other species or go around destroying life needlessly, but that killing off humans is much worse than killing off bacteria, from a human perspective. Your assertion to the contrary is lunacy.
We're capable of thought, and bacteria aren't. That's a pretty sharp divider. I'd be grumpy if scientists were going to wipe out a planet of earth-plants, but it would be a travesty if they wiped out the planet of moose.