It's a web API. Browsers implement it. Including Safari and FF. If the argument is that it's hard to implement and makes it harder for new browser implementations, the same could be said of new CSS features, other APIs like Web Audio, etc. Trying to brand Web Components as Google-only may have worked in the v0 prototype days in like 2014, but it's factually not the case today.
This whole project appears to be a security disaster and nobody should use it.
Web applications are fully isolated and sandboxed, have fine-grained permissions, are easy to inspect, and the runtime is built with a modern threat model.
ChromeOS is probably the most secure desktop OS for this reason.
I want my browser to expose more functionality to web apps, because it means that I have to run less random unsandboxed code on my underlying OS.
Until they're delivery vehicles for obfuscated wasm to canvas rendering applications. Then nothing of the "web as graph of hypertext documents" will be left.
(also, wasm changes nothing here, you could always obfuscate js just as much)