We definitely shouldn't give up on averting the catastrophes we know just because we've got a plan B but at the same time we shouldn't give up on a plan B just because we think we can avert a particular catastrophe.

At the end of the day "self-sustaining civilization in an already inhospitable and isolated place" is probably more about showing we've already managed to find a way to eek out under pretty much any situation than it is particularly about Mars itself. To be honest life on Mars would be significantly crappier for probably decades upon decades - but at least we'd prove out the floor and our ability to raise it over time instead of assuming we'll figure things out before our assumption where we are will always be supportive to us falls through for whatever reason.