Plain old HTML stops being responsive as soon as you try to do anything more than text. A plain HTML table would be horrible on mobile. If you want to reflow anything that isn't text you will need flexbox. There is literally no reason to not use CSS to make a site that works well on mobile and desktop.

I’m not convinced that’s true. Case in point: tabular data. I don’t think presenting something in a table is an attempt to make it look good, I think it’s the most logical and recognisable way of presenting certain kinds of information. But tables don’t scale well on a mobile viewport without extra work.

Reasonable tables work fine on a mobile viewport. It’s when you get into crazy ones that it breaks down.