If you're pulling from npm, you should be getting the already built version. And to use it, you can use a regular function expression instead of the fat-arrow syntax. I'm not sure what requirement you're referring to.
The project itself is using wp/babel, but the output is usable, and what's in npm that you can use without it. All the dependencies in package.json are dev dependencies, if you do a prod install, it won't even download them.
That said, I can't think if the last project I started that wasn't using babel.
Ah I wasn't aware that the built version was published on npm. Thanks for pointing that out. I downloaded the library and ran the build process myself. NPM install pulled in about 100MB dependencies. This seemed really overkill for a 30 LOC mini library.
IMHO the majority of the web does not use stuff like Webpack / Babel (yet). Just look at all those Wordpress, Joomla installations and apps older than 2 years. So being able to pull in a frontend library via script-tag is still a must in my opinion.
Yeah, though npm is really an ecosystem built around cjs modules (though some moving towards ES6), and a lot of code now being written in ES6. In some cases, it's hard to decide to leave the ./src directory or not, in addition to ./dist for the potential of tree shaking etc.