As far as I can see, there are very, very few disasters that would render Earth less inhabitable than Mars but not also destroy Mars in the process. Even if Earth were reduced to a cold, lifeless rock with almost no atmosphere or water, high radiation levels and almost all sources of energy gone (including geothermal and wind energy and things as simple as burning wood), that would just put it in the same ballpark as Mars.
Basically, if you have the technology to support human life on Mars, you have the technology to disaster-proof Earth.
The concern is not a disaster that necessarily destroys Earth itself - an extinction event for humans is enough. There are plenty of species that have gone extinct before (including some close relatives) and since it's obviously never happened to us we have little idea of its shape or likelihood.
The very distance that makes Mars so hard to reach would be good protection against such an unexpected event (whether it's unintended effects of scientific experiments, novel WMDs, grey goo etc.). There is no way to create a similarly isolated colony on Earth, even if we discount the problem that nobody would want to live in such a place. (As opposed to the many people, reared on SciFi, who would volunteer for Mars quite eagerly.)
The scenario that comes to mind is a massive asteroid impact, but your final sentence sums up the objection nicely: identifying and redirecting a space rock seems at least an order of magnitude less difficult & technologically distant than building a self-sustaining colony on Mars.
And yet, again, a Moon colony would be handy to counter an asteroid too. The far side could host very powerful telescopes that could map NEOs with unprecedented efficiency and clarity. On Earth, you have a couple hours of sensor exposure. On the Moon, you have two weeks to trace stuff. And most of the heavy parts could be manufactured in situ, be lighter and higher quality (made in a high grade vacuum).
And that's to say nothing about the ability to launch spacecraft from a lower gravity and/or using a magnetic rail. One could even turn the asteroid into dust (which is less dangerous than a ferrous mountain falling from the sky) with a kinetic impactor packing enough energy.
If you're going to survive on Mars, you already need the ability to sustainably disconnect yourself from the surrounding atmosphere (we don't know of any virus that's as lethal as lack of air), so just apply your putative Mars tech to Earth.
Food shipments? Water supplies? We're talking about the kind of technology that would allow you to live on Mars. If you need to rely on those, then 1) Your Martian colony is not really independent and thus doesn't carry any survival benefits, and thus 2) You're wasting resources shipping them to Mars. If you don't need those for your Martian colony, then why would you need them for your Earth colony?
If Earth were in a situation of having to segment populations to prevent the spread of disease, all it takes is one little mistake to wipe out an entire intermingling group of people. I'm not saying we wouldn't have the tech to prevent that from happening, but humans are fallible creatures. Is it worth risking the entirety of humanity when we could have a remote colony that would be very well protected from those types of events?
Very well protected? The danger specifically from viruses might be somewhat less (though if we're allowing people to travel there, it doesn't seem like much less of a concern), but any other danger you can imagine would be much greater. The remote colony would be no more protected from disaster than one on Earth. On the contrary, since the surrounding environment would be irreconcilably hostile, it would be constantly at risk of one little mistake wiping it out.
The odds of extinction within the next 50 years are still low. A self-sufficient Moon colony would cover most of the disaster scenarios adequately while being a lot cheaper to build than a Mars colony. Besides, having a self-sufficient lunar colony would probably make building a Mars colony much cheaper.
On the Moon, solar energy is abundant (close to the Sun, no atmosphere) and so are the metals used in spacecraft construction. Without atmosphere and with a low gravity, launching cargo to Mars is easier and can be done with a magnetic rail launcher, effectively using solar power to launch a spacecraft to Mars.
So, I'm not that guy, and really trying not to be that guy, but wouldn't it be fair to summarize your general point as: "That's no moon. It's a space station." ? (mainly mentioning it because it could be taken for an inspirational saying in this context)
> Insurance for humanity against disaster is a small ROI?
A self-sufficient colony on the Moon would be efficient insurance against most disasters I can imagine and much easier to build. A colony on Mars offers negligible benefit over one on the Moon for the purpose you mention.
