Well, it's a more reasonable explanation than aliens at least.
A. Aliens
B. Piece of one of our own spacecrafts
I choose C. None of the above.
Is it perhaps the more comfortable of the two possibilities?
The more plausible of the two (Occam's razor). Even though it's highly unlikely to stumble on a spacecraft wreckage on Mars. It is even more unlikely for this to be an alien tech. Though, I do admit, I want it to be :)
As Douglas Adams said:
"Man has always assumed that he was more intelligent than dolphins because he had achieved so much...the wheel, New York, wars and so on...while all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man...for precisely the same reason."
Does it not strike you inappropriate to do so considering the billions of years of evolution and absolutely random events like mass extinctions that led to the rise of mammals and eventually humans?
No planet will have infinite resources, so it's logical that life will develop through continuous competition for territory and materials--just like it did here.
Looking back at Earth's history, it's not like we are the only violent forms of life we know of. Quite the opposite.
Further, any microbe hitching a ride aboard an alien may well find Earth's general environment to be easy to thrive in. Think Kudzu, but at the microbial level. Eventually, the alien microbe and/or the natural Earth microbes will mutate in order to out-survive the other.
Anything really dangerous, like something that can infect multiple species, would have to be "engineered". "Mother nature" (aka evolution) doesn't engineer bio-weapons made to take over alien ecosystems - you would need huge time spans plus ubiquitous means of interstellar transportation for natural selection to favor such features (think "star gates" but zillions of them with tons of creatures and cargo running through them for at least tens of millions of years).
So I'd take contact with a "non-engineered super infection" like you describe as a sign that someone has been operating a huge network of interstellar transportation systems for quite some time, as this would provide the only "peacefull" evolutionary history (again, excluding civilizations expert in bio-war things...) for such thing to evolve, so it would be quite a good omen for hopes "intergalactic fraternity" :)
As for microbes from other worlds, who knows. The properties of water are pretty much required for life (as far as we know) and so they'd have 0 trouble interfacing with most organisms. Combine that with the fact that the basic amino acids can form in a wide range of conditions and you've got the recipe for interference. Also, infections happen when organisms invade and multiple, some times the damage is a secondary side effect (i.e. waste or toxic byproducts)
Name the occurrence in the history of our planet where the much-more advanced/capable species did not displace and diminish the indiginent one. In many cases, the more advanced or aggressive species completely wiped out the existing one.
There's a lot more that's extremely unlikely in the Star Trek universe besides just transporter beams and faster-than-light travel.
A sufficiently advanced species might not feel any more compunction about eradicating the human race than I would feel about eradicating mosquitoes.
Name the occurrences where they did. That word, species, I'm not so sure you know what it means.
Huge, unfounded assumption.
I think that's the point. When you don't know anything about the motivations of an alien race that can easily wipe out our species, why would you take any chances?
Pretend for a moment that I have a Button. The owner of that Button can push it and eliminate every person on the planet besides the people that owner didn't want to kill (friends, family, city, state, country, whatever)
Would you be an advocate of giving that Button to any random person on Earth?
Any race of aliens capable of interstellar travel probably has that Button. So while (I assume) that you wouldn't be okay with giving that Button to any person on Earth -- you'd be okay with having an alien race that we don't understand even a little bit in possession of such a thing.
We are all aware of the reasons to be pessimist, because of all the books and movies, but there are reasons to be optimist too: advanced technology may come with advanced ethics, for instance.
You criticized the original poster's sense of pessimism for holding the only parsimonious, evidence-based and logical position on the subject.
Have whatever fanciful opinion you want to on the subject, but pretending it's based in anything but wishful thinking is naive.
Unless of course you believe they're hiding inside Europa...
Hawking has said that we need only look at the history of inter-species relationships on our planet (let alone race relationships) to get a hint of what exopolitics might look like. He goes on to mention that in terms of the search for alien life, we'd do best to keep our heads low.
I'd rather compare alien encounter with something like Marco Polo going to China: he didn't wish to kill them all, was impressed by some achievement and inventions but was also disgusted by the Chinese diet. Came back and wrote a book, and it was so foreign most readers didn't believe him.
And there is a reason to believe that they probably won't be - they will be the product of Darwinian evolution, just like us, which is really not a very nice thing.
Because of the properties of exponential growth, there would be nothing left over for us. If two organisms double their numbers 250 times, their numbers would roughly equal the number of atoms in the observable universe, which means there would be none left for anyone else.
The only hope would be that they either don't reproduce, which seems unlikely since they would be the survivors of a process of Darwinian evolution just like any other critter, or that they're Space Vegans with a moral code against exterminating other species. We really don't know how they would think.
Discovery of alien would be nothing but revolutionary for the human race but don't let scifi and fantasy get in the way of reality. A rock formation like this is not unheard of just because you've never heard about it.