In and of itself, the attempt to colonize Mars probably won't do much for the species. The value is in actually going through the paces: developing primary technologies, techniques, protocols, and doing research, especially observing how organisms (ultimately, humans) are affected by extraterrestrial life, etc.

If the worst comes to pass and the species needs to throw a hail mary, I have to think our chances of success would be improved (even if slightly) if we were to have these sorts of details ironed-out in advance.

There's also the terraforming possibility, even though that's still a long, long way away. It may turn out to be easier to create a habitable atmosphere from scratch than to make even relatively small changes to our own, since the Earth already has all these complex feedback loops. There's also not as much margin for error here because, as The Tick would say, this is where we keep all our stuff.

Not going to be a problem in the next 100 years, but ...

Suppose we started to really use Fusion power not just at our current energy needs, but enough to directly warm the planet. There are hard limits on just how much heat per m^2 you can dump before you start to cook the planet.

Such a civilization may prefer low atmospheric pressure and a planet further from the sun simply for cooling.

I agree. However, I doubt anyone who is actually investing resources in getting to Mars is doing so with climate scenarios as their sole motivator.