I've noticed a lot of large comments written by the historians don't include a TL;DR. I know historians put a lot of work in to writing up their work to help us non-historians, but at the same time, many of us don't have the time to read it all, whether we're at work or school, it's easier to have a small TL;DR paragraph than a 5 paragraph page.
I mean we can't really force people to do this. It's up to the individual commenters to include it. Some do and some don't. Some questions really just don't warrant a tl;dr given the complexity or nuance involved, so it wouldn't be doing anyone any favours to make it a rule. And like you said, sometimes a full paragraph is needed for the tl;dr, but then people just end up complaining about that (since there seems to be some unwritten reddit rule that a tl;dr can only be one sentence. It's silly but I see it brought up a lot).
I personally try to include one, but it's usually just some flippant single-sentence thing which mostly only exists to catch the few people who might otherwise not read anything on the page, so that at least there can be some eye catching thing to help curb further spread of misinformation.
Speaking just for myself, I guess I'd say that if one really cares about the question being asked, then reading 5 paragraphs should be something they're happy to do, and demanding it be compressed into 1 isn't really the thing people should be coming to AskHistorians for. Brevity kills nuance, and nuance is what most topics here really need.