My take is that we're never going to see ES Modules in the wild in web apps as we know them today, outside of some tutorials or open source apps. There's too strong business incentives to the contrary, legitimate or not.
(that's a legit question – i've sifted through webpacked JS a few times, and it mostly just added a bunch of
(function(module, require, ...) {
<your module here>
})(...)
and i guess if ES Modules turned out to be more performant (bc the module stuff is handled by native browser code, or maybe some HTTP/2 magic with loading), that could be an incentive for adoption