I love the clean approach with classless. Documents do have a structure and is makes it easy to change by just swapping out the CSS without touching the document.
But could you not just add the class only when you really really need to break the structure? The middle ground for me would to do my utmost to avoid classes within reason. So as few exceptions as possible. I know this is selling elastic bands by the meter.
On the other end you have Tailwind CSS. I know many are happy with it and find it has a nice developer velocity. But I find it overly verbose and "framework of the day"-ish.
So for me it is classless until my head hurts. Then I'll sprinkle in a class to get work done.