The entire blog post being in lowercase distracted me more than I thought it would.
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I think many of us abandoned it when we went professional. Or abandoned it in those contexts but still do it in others. I don't do it on HN, clearly - but I do it almost everywhere else. It's much more natural to me to skip capitals.
I believe there was also a period in the transition to ubiquitous smartphones where it wasn't an option to turn off auto-caps, or maybe there just wasn't the fine-grained control of which auto-correct you use on mobile devices that there is now. I suspect that killed some all-lowercase habits. I think that's why I ended up with a "normal" style on HN where I use caps and normal punctuation (I don't usually use periods for sentences that terminate a paragraph outside of HN.)
For me it wasn't about being professional, it was just about learning to type. As my typing speed improved it just became second nature to capitalise where appropriate. In other words, I capitalise everywhere out of laziness.