Partly I agree, 5% of users is a lot.

However, this number will only continue to decrease as time goes on, and it also has to be questioned for any specific product whether that number is bigger or smaller on average.

Both for the (near?) future when ES6 is near-universal and for projects that have the luxury of just ignoring older browsers even today, it would be nice to have a proof of concept project like this that uses modern ES6 features, since many of them address precisely the types of problems that many pre-processors also fix.

Template-strings, custom elements, etc. add a huge amount of possibilities for web-application development and imo should get way more attention outside of a small circle of excited people.