Are you genuinely asking as a professional? Seems like a big ask for someone to go over all you are misunderstanding if you think they are equivalent to a template language.
There are a lot of candidates for “what’s wrong with modern web standards” but this fragmentation, which comes from a rather mathematical view of programming, is one of them. Thing is, a lot of web devs never studied computer science (even CS 101) and less than 5% live in San Francisco.
How will they ever understand web components.
1) You dont have to load an external library
2) Shadow DOM
3) Dynamic slots
That’s about it, honestly LOL.
I guess the main point of most browser APIs was to let apps use browser features.
This one actually tried to make a standard way for apps to use other apps. But they already had their own libraries so nyeh, thank you very much! LOL