From my perspective, it is less about what we can use in the browser without transpilation, but where the js ecosystem is heading. I personally only want to write JS that is in spec because in 5 years something like decorators might not be a thing. Basically when I'm figuring out what language features I care about, I'm thinking about how hard it will be for other engineers to work on the codebase. Things that are not in spec are going to be less familiar and more time to learn. I think there's a lot of momentum with the spec and something to point to where can say "you should learn this syntax/feature because it's in the spec."