HTML is not for writing but for creating UIs. That was why it was created anyway. Markdown was specifically created for writing. It has no other purpose.
Saying that Markdown is bad is really naive. There are not many ways to specify formatting without it getting in the way of your writing. Markdown is popular because it strikes the right balance where reading a markdown document requires a lot less visual strain compared to the same HTML document. It requires the least amount of letters to specify formatting - italics is 2 chars, bold is 4 chars, underline is 2 chars etc. Lists are mostly inferred and require no additional formatting. Compare that to HTML which requires 5+n chars (where n is the number of chars in the tag name) regardless of which format you want to use.
Markdown does have it's quirks but they don't come up 99% of the time unless finding them is specifically your intention. Knowing whether *__hello__* is bold italic or italic bold is not important unless you are writing a parser.
The downsides of Markdown are much, much less than the downsides of HTML. Writing is not the same as designing a website or a UI. It's not meant to be pretty and typesetting + styling should not be the concern of writers.
Html was absolutely created for writing, though it later got hijacked as a system for creating UI's inside of the browser doing the browser wars of the 90ies, but if you look at the original specs, it's a way of marking up documents and it was very much readable even without a rendering engine.
Prior to html and the web revolution people would use different macro packages for either troff or TeX and, and in the first years of HTML people would absolutely write it by hand thinking very little of it but then again that was before style elements css and javascript.
After the browser wars almost everyone who working with documentation had their own perl, or ruby scripts to take some simplified markdown syntax into html and there is of cause a risk of markdown itself getting bloated to the point where people need scripts to turn simple meta formats into correct markdown.
Saying that Markdown is bad is really naive. There are not many ways to specify formatting without it getting in the way of your writing. Markdown is popular because it strikes the right balance where reading a markdown document requires a lot less visual strain compared to the same HTML document. It requires the least amount of letters to specify formatting - italics is 2 chars, bold is 4 chars, underline is 2 chars etc. Lists are mostly inferred and require no additional formatting. Compare that to HTML which requires 5+n chars (where n is the number of chars in the tag name) regardless of which format you want to use.
Markdown does have it's quirks but they don't come up 99% of the time unless finding them is specifically your intention. Knowing whether *__hello__* is bold italic or italic bold is not important unless you are writing a parser.
The downsides of Markdown are much, much less than the downsides of HTML. Writing is not the same as designing a website or a UI. It's not meant to be pretty and typesetting + styling should not be the concern of writers.