alt Hacker NewsThere's a difference between something being extremely difficult and it being actually impossible.
And yes, Mars is not going anywhere, but the same cannot so confidently be said about our ability to get there. Our species has been capable of spaceflight for a tiny fraction of our existence and there is plenty of precedent for catastrophic loss of knowledge and capability.
Mars is not a safeguard against Earth going to hell, even in the most extreme Mars fantasies you don't end up with something independent of Earth for an indefinite period of time. We're also more than capable of exploring our Solar System in immense detail without sending people or colonists. Our greatest achievements on Mars have involved rovers, and frankly suggest that as a planet it has little to recommend it.
tl;dr There is no reality in which it's easier or more reliable to "back up the species elsewhere" than it is to act in our interests here. That doesn't imply shutting ourselves off from space, but it means treating it more like the deep ocean where rovers and drones are simply a better fit for such hostile environments. I'm hopeful that will change someday, but anyone who's thinking in terms of decades doesn't understand the scope and depth of the problem.
My recommendation is to read some of the books and articles suggested in this thread before imposing your wishes on reality.