Do you have a more substantive reply to the user you’re replying to? These are genuine observations/experiences. If you disagree or think they are misguided, then point them out.

The issue is that the main comment in the thread complained about design, but gave little concrete information to discuss. If I come and say "Design matters, and HN's design sucks. Go and compare it to Reddit/whatever other site," you likely will have little idea of what my beef with HN is.

Of course it is. It's usually avoided depending on the level of formality/style of the writing, but there is no grammatical rule that says a paragraph cannot be one sentence long.