We can stop transpiling to ES5 when a long enough time-window (one you're comfortable with) has elapsed since the last ES5-only browser was released and hopeful

We can stop transpiling to ES5 when a long enough time-window (one you're comfortable with) has elapsed since the last ES5-only browser was released and hopefully your users have upgraded. Additionally, you can check the User-agents in your access logs (or analytics) to see what fraction of your users are still on ES5 browsers. I think once you've correctly configured your CI transpiler, you wont even have to think about it, so it's a small price to pay, IMO.