Yes for speed purposes. And yes some people may not like types, that's why I said completely optional. Anyways the optimizer tries to figure out the types on it's own, so give it a non blank slate to start with. You are correct in noting that js is already fast enough, but the problem is the sort of performance gotchas(order of keys in objects) as mentioned in the post aren't really obvious during development time. Types may help level the unpredictability.
I'd say it would be a lot easier to simply focus on the WebAssembly format, you can write your code in a number of (typed) languages and compile into wasm which is designed to be fast.
Javascript already comes with a few decades of bagage, and things are already changing really fast (ES6 and newer versions). Not everyone likes types and the people who do can use typescript (or flow). And the people who need insane performance (games, etc) can use WebAssembly.