I've asked the same question to many professional bloggers (I run a platform for bloggers).
"Did you use markdown to write blog posts". The answer was a big NO. Most of them use the default WordPress editor (now Gutenberg). Some of them use Google docs and other 3rd party tools.
As you said, markdown is mostly used by developers for their blogs. I haven't seen many professional bloggers not using it
I'm a developer and a blogger. I still don't use markdown :D while blogging
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I'd say Markdown is becoming the standard of writing on the web. If the client doesn't want to write in Markdown, they can use some tools which can help them convert the text to it, but overall it's their problem. Static websites have a large base of companies using them. E.g. Middleman was used and developed by Mailchimp.