I don't think this is generally true. The purpose of long form writing is to make the reader draw a conclusion that they may not agree with at the beginning of the article. Studies have shown that once we form a conclusion, we're much less likely to change our minds, so if an article lead with a controversial point we'd internally say "no!" and stick to it rather than letting the article make its point. By not knowing exactly what an article is arguing, we're more receptive to things that could go against our intuition.

I do agree that TL;DRs can be elided for articles that simply state facts or uncontroversial ideas.