it makes total sense because native implementations can have higher garbage collection optimizations than library's in JavaScript land.

This is exactly like promises. Everyone said "why add them to the language, we can just use bluebird/Q/etc.". Luckily, they were ignored, and the tooling around promises is much better. We have a standard primitive, and everyone is happy except some monad purists. People just hate change. When literally dozens of libraries have all converged on a similar reactivity model, it makes sense to codify it into the language so that people can stop wasting their life with duplicate workloads. You'll be able to use libraries in concert without worrying about conversion, and diagnostic tooling will be built into the browser. Everyone wins.

It makes very good sense