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.
The standard, like PWA, is designed for maximum feasible misunderstanding. To the average dev it seems like a random bunch of features that don’t hang together. Five developers could look at it and be like the blind men discussing the elephant —- hung up on individual parts and not seeing the whole, if there is a whole.
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.
On the other hand, if they're completely different from a template language it seems like it should be just a moments work to demonstrate why, and help a fellow professional understand what they're missing.
Now I'm curious, what template engines don't have dynamic slots? Is that actually a rare enough feature to declare webcomponents special for having it? Does webcomponents have an advanced version of it?
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.