If you make such risks up, then no one says that html will be readable in the same way in 20 years (which is already kind of true as you have a lot of old blogs with awful layout on mobile)
But there is an even easier one - nothing will happen to your html built files of 20 years, they'll remain... static.
The worst is you'll go to updating your lovely html by hand, but you would've saved yourself wasting 20 years doing that instead of using a better system.
I promise you, HTML is not a hassle... It's markup. You describe the page, and then... you're done. And if you don't use weird, niche tags, it'll work basically forever.
This is a patently false promise. Yes, it's markup, but a bad one un-ergonomic-suitable for direct human consumption. Even the author of the blog understands it and uses alternatives
> with md-block for markdown
> You describe the page, and then... you're done
Unless you want to re-describe something, of course. Maybe sort a column in a table...