I find it more compelling than I expected.
The truth of the matter is that as much as the idea of separation of html vs css sounds good, it's so hard that I rarely see it done. Without constant vigilance, you end up with premature abstractions, things you can't actually reuse, and an append-only accumulation of CSS.
The dream sounds good but leaves all the hard details for you to work out yourself. And frankly, after over a decade, I remain unconvinced and ready to look for new ideas.
Breaking the rule with Bootstrap since it first came out never stopped me from building great things, yet trying to obey the rule slowed me down and had me constantly refactoring CSS. It feels like a cargo cult.
We've been pointing to CSS Zen Garden for 17 years as an example of the benefits, but giving everything a class so that you can address it from a .css file isn't the hard part, and that's all the garden shows. The hard parts are organization, maintenance, coming up with a unified strategy, playing CSS whack-a-mole when you change HTML, deciding where to define what, finding where you defined what, etc. The garden demonstrates none of that.