I have been wondering about this a lot. It feels like about a year since writing then transpiling ES6 became a fairly mainstream approach, and we're nowhere near having full ES6 support in enough (or actually any, for now at least) browsers with a high proportion of usage.
This is manageable because you can just reason "well, I'll transpile everything to ES5" and be done with it. But what happens when most browsers do support ES6, but not ES7, or ES8 - what about when people don't update browsers, etc. I am really worried about this becoming such a nightmare that the default will always to be to transpile everything to an older version (probably 5 or 6) for the next 2-3 years just for safety, even when some browsers do support new versions coming out.
Can anyone point me to any resources which address and discuss this issue? Your idea about specific browser version targeting in the transpiler is a great one, and one I'd not even considered before, thanks.