Fetch looks interesting but as far as I can tell, it's an in-progress standard with no support on any version of Safari or IE or any mobile browser whatsoever. Seems like it has a bit to go before it being 'native in modern browsers'
You can polyfill it in those browsers. Just use polyfill.io, or if you prefer, manually feature-detect it and if necessary load Github's fetch polyfill, which is good: https://github.com/github/fetch