Such a system would quickly be gamed by the downvoters who would start marking things they disagree with as 'spam' so another layer would need to be added in which certain types of downvotes - spam, flamebait and other such categories - are checked and undone if needed. Downvoters who abuse their voting privilege too often would loose it for a day, then a week, then a month, etc.
In other words, something like the Slashdot moderation/meta-moderation system of old.
Appart from obvious nonsense, the typical cases for down-votes are not descriptive (what is true or false), but normative comments (what should or should not be the case). Normative comments, such as what user `blueprint` has written, have the tendency that people up-vote or down-vote according to their own, often quite ad-hoc opinion, using the system like a poll. I think that this is not good. Instead, one should either just ignore it or try to initiate a debate. For the vast majority of opinions that contradict one's own, there are plenty of representatives out there who are at least as clever as oneself. Being curious about why they have such a view does not have to mean that one relativises everything. There is almost always something to learn.
I also think that we should strive to apply the principle of charity[1] in our responses, mending small flaws or bad rhetoric in a person's statements and instead challenge the essence of her or his thoughts. Although user `blueprint` does not elaborate on his topic, the topic itself is quite interesting. He just worded it a bit clumsily. Its essence may be reformulated as: "Given that an extremly devastating, but also extremly unlikely cosmic event could wipe out humanity in an instant, should we not prioritize to spread out across the galaxy, so that a single event could not affect all of us?" Has this statement the same chance to be down-voted? If this is not the case, doesn't it mean that a down-voter should have applied the principle of charity instead?
[1] Wikipedia's definition: "In philosophy and rhetoric, the principle of charity or charitable interpretation requires interpreting a speaker's statements in the most rational way possible and, in the case of any argument, considering its best, strongest possible interpretation." -- https://en.wikipedia.org/wiki/Principle_of_charity