The idea is that if you can't get your quote published outside of Wikipedia, you shouldn't put it in Wikipedia. Preferably, "outside" shouldn't be your own blog, but something more respectable.

The conflict of interest part is (this is my opinion, not Wikipedia policy) just a simple heuristic to prevent most of the "it's not anywhere else, but I insist it should be in Wikipedia" edits.

Yes, it is extremely annoying if a newspaper prints a false information, and some Wikipedia editor insists on adding it to the article, and calls all expert explanations "original research". But still, the heuristic in general is useful -- it would be much worse on average to instead have hundreds of crackpots claiming expertise and "correcting" information from newspapers and books. It is easier to find a third-party volunteer to verify whether X really was published in a newspaper, than to verify whether X is true. -- And you need a lot of volunteers who don't have life and only care about the Wikipedia rules, because if you remove all such non-experts from the game, and leave only the experts and the crackpots, the crackpots will obviously win by their numbers and dedication.

If a newspaper contains an error, the long-term strategy to fix it is to find or create another newspaper article or a book that will correct the error. In general, the battles should be fought outside of Wikipedia, and Wikipedia should just announce the winners.

The only interest I have that could disqualify me is that I don't want humanity to die.

Problem is, there are many people who believe the same. Don't only think: spam, but also: religions, cults, homeopathics, etc. Even if you are right and they are wrong, the difference is only seen by those who agree that you are right. For everyone else, this is just one of literally thousands of different causes that wants to be promoted by Wikipedia. The Wikipedia immune system will evaluate you as a threat.

The Wikipedia system doesn't work in a way that effectively identifies all suitable texts.

Well, this is the place where you have the chance to add the text successfully. The more important and relevant the source of the text, the higher your chance of success.

I'm surprised that given the amount of utilitarianism on Lesswrong that sentiment doesn't get a better reception on Lesswrong.

Okay, let's talk about consequences. You add a MIRI quote to Wikipedia, someone deletes it. You add it again, someone deletes it again and quotes some Wikipedia rule. You add it again and perhaps even say that you consider Wikipedia rules irrelevant in this specific case. The quote is removed again, and now you have a group of people who have no life watching the page all day ready to remove your quote if you add it again. Also, you have increases the probability of other MIRI quotes being removed in the future, even if they are moderately well sourced.

Strategy B: Get the quote in some high-status source. Then add the quote to the article, somewhere at the bottom (obviously it's not a part of the plot, nor the characters and cast, but maybe criticism), with a reference to the source. The probability that it stays there is much higher.

I guess it's easy to argue in the abstract that one should push the fat man

But in real life the fat man has dozen deontologist bodyguards ready to stop you. So instead you listen to the bodyguards; they tell you they only obey the wisdom from newspapers, so you bring them the newspaper article recommending to push the man, and they will happily push him themselves.