I've never considered #9 (global namespace) until yesterday, but it seems like a show stopper for dynamic web components, whereas a system like React has components simply being JavaScript objects that can be imported and whose name is largely irrelevant except at JSX (or HTM[1]) parsing time.
It's being worked on: https://github.com/w3c/webcomponents/issues/716
The fundamental difference here is that HTML needs to know what class to instantiate when it sees a tag. Supplying that unlocks a lot of power, like custom elements in the main document, Markdown support for free, easy integration with existing CMSes.
We can and will solve the single namespace problem though.
By the way I just noticed you're the maintainer of lit-html. How does it compare to HTM[1]? They both look like they accomplish the same goal but HTM's implementation looks smaller.