I always saw WCs as the leaves to the root that CSS frameworks already offer.
You can drop in Bootstrap and the whole page looks nice, but the moment you want to use the GUI elements that consist of multiple HTML tags, things get ugly.
WC would offer a way to merge these into one element.
Then you could drop your JS framework of choice in the middle and be done with it.
React isn't cool anymore? Well, at least you can take Bootstrap and its WCs with you. I had the impression CSS frameworks had a longer half-life than their JS counterparts anyway.
I don't really care about all that HTML module stuff or JS requirements. Sure would be cool if things were different, but from a practical POV it doesn't seem THAT important to me.