Zipf’s law (rank frequency distribution) usually gives a workable first guess at these things.
If there were four or more successive levels of the operating system active in the wild, you would normally expect the distribution to roughly halve at each level:
1/2 + 1/4 + 1/8 + 1/16 etc etc
So even in two years, it will still be sensible to paste-copy any code to be shared through Babel JS, to bring it down to a more widely shared ES5 base level.
i.e. nothing stops us from writing in ES6 and sharing in ES5 now – there’s no particular value in waiting.