We could use his coffin/capsule for it. A sad, but poignantly dignified monument that shall serve as a warning to others. I suggest the epitaph: "We repeatedly told you that Mars was uninhabitable."
Edit: or perhaps more relevantly: "well, we tried to but you fired us."
It's not a matter of never try, it's more of a matter of what's even practical. Inhabiting Mars is not as simple as just landing a rocket on there with a bunch of people and materials. Space travel requires immense physical training, enormous costs just to get into space, plus there's the issue of handling emergencies if anything goes wrong while on Mars.
It's an idea that's so far off in terms of the technology that we'd need, and there's so many more useful things that are closer to within reach that are still similar pursuits that would be more valuable investments (i.e. asteroid mining, advanced satellite technology, etc.). Making incremental progress is great, but there's still the question of "what would we even gain from going to Mars?". There's no ore that'd make sense to mine, making/terraforming a civilization there when we can't even make one in Death Valley (which already has oxygen) is preposterous, and tourism would be impossible due to the physical limitations of space travel.
It's not that we should "never try", it's that there's no practical reason (right now at least) _to_ try.
You could go back in time before other countries were discovered and make a lot the same arguments about heading out into the ocean on a boat.
Technology will never get to the stage it needs to in order to say live on Mars (or anywhere beyond earth) unless we actively venture out and try to do so.
I think that's the point. It actually doesn't matter if Elon fails in getting anyone to step foot on Mars - by believing it to be possible, he's creating a kind of self-fulfilled prophecy.
Without anyone trying there's 100% chance it'll never happen, and by the time you need it to happen it'll be too late.
Here's a difference: you or I could get on a boat with little difficulty besides sea sickness and wobbly legs. On a rocket, you or I would die or suffer from other physical ailments caused by simply being in space and in different gravitational environments for extended periods of time. This isn't an unknown, it's a known.
On the ocean, you could land on a island, fish at sea, or be lucky and have rain provide water. In space, you have nothing, and guaranteed nothing for weeks, months at a time. Again, this is not an unknown, this is a known.
We're just simply a large number of significant innovations behind where going to Mars is unfeasible, physically and monetarily (namely, human physical/mental limits in space travel, time, supplies/oxygen, emergency response, funding (think of how expensive a single un-manned mission is), etc.)
It would be akin to telling the vikings to make an airplane. They would first need to discover engines, improved metallurgy, electricity, and a thousand other things before it would be possible and practical. The idea of a flying machine has been around for thousands of years, but only in the last hundred or so was it actually possible, and only the last 75 or so practical for an average commercial person. And even then, airplanes can always get more oxygen because they're within Earth's atmosphere.
To make one thing clear, I'm excited about the prospect of interspace travel (how could anyone not be?!) But, Mars as a goal is _so_ far off that it obscures and hides the reality of the steps and innovations that we'd need to make along the way before we can seriously make an effort to do anything productive on Mars that wouldn't be easier, cheaper, safer, and more effective closer to Earth.
