I know this is the way it's going but for me it's a bit strange to discard CSS or change it's range of influence to JSON.
I think I understand the flexibility but is it best for a developer?
CSS will (should?) always be the style file of the web - which, for me, has proved to be a solid HTML + CSS + JS + Server Side Language (PHP :) 'block pattern'.
I imagined it could be easier to break [CSS] it into different levels of utility (Core, Theme, Pattern, Edit, Blocks, etc?) and then WP would bundle it as necessary.
I would also mention that a Theme for me acts more like a behavior controller of dynamic content and a Pattern a layout controller of blocks.
When blocks get dynamic (fully playing with the database) maybe they become one..?

Sorry for these messy thoughts but I'm not that much inside WP architecture and I find these moments so interesting to learn.
And this block Era a particular one.