Indeed CSS is broken. But, if you tried to make that case several years ago during the Great War on Table-based Layout, you'd have been absolutely persecuted. Oh how we wrangled with the utter pain that is CSS layout, only to appease the tableless zealots. I can remember wrestling for hours with simple layouts that just wouldn't cooperate cross-browser. Meanwhile, I could achieve those layouts in mere minutes with tables. Not only that, the resulting table-based design HTML was an intuitive read.
The reason was as you say: Grids are key.
How many people would now honestly say that we are better off for CSS-layout vs. tables? But, it's amazing how an entire industry lurched in one direction so quickly, based on ideology. The pressure to use the "right" approach was immense. The whole thing had a very "Emperor's New Clothes" feel to it. Everybody seemed to agree that CSS layout was completely awesome, while I beat my head against the wall trying to make it work. Now, we all see that so many were actually suffering in silence.
>the separation of concerns in HTML into content/style/controller is very useful
This is a belief that I question to some extent and I've wondered whether it's a core assumption that sounds great and thus goes unchallenged, but fails to make the cut in the practical world. In particular, how much do we really need to separate content from layout? Styling is one thing, but layout tends to be intrinsically tied to the content. You can change the appearance of content without modifying its meaning, but changing its organization or layout can render it absolute nonsense. Thus, if CSS were all about colors, fonts, etc., it would actually be a somewhat reasonable approach to separating reasonably separable concerns.
But, as most now agree, it's CSS's horrible attempt at managing layout that makes it so painfully bad. And, I really wonder if the core problem is just that there's no real benefit to trying to separate layout (vs simply style) from content in the first place. What we're really saying in attempting to do so is that we want to be able to create a document wherein we have a bunch of unstructured content that we want to insert in an essentially random fashion, then create a second document that rearranges the content.
It's an unnecessary abstraction that seldom provides an ROI. How often do we later want the content to stay exactly the same, but simply want to change where it appears on the page? And, if we do, is it really easier to change the layout in an external document (CSS or otherwise) than it is to modify the document?
I have yet to hear a proposal that makes more sense than the old-table based design. The purists said that tables are strictly for presenting tabular data. Fine. So, let's create a new tag called "grid" for layout and give it the same layout flexibility as the tables of old.