Suffice to say, I've run into the exact same issues as the author. Had they allowed me to contribute in a meaningful way Elm now would have a nice UI library and probably a much nicer package website which I've developed as well [3] (no longer running, but I've turned maintenance mode off for a few days so you can take a look).
I stopped using and contributing to Elm when 0.19 came out. If you are interested how much effort I put in just check the Elm related repositories in my Github profile [4].
After that I missed the developer experience and I created Mint [5], so in the end some of knowledge I gathered during that time have been put to good use :)
I just can't understand how they could ignore all the work I've put in, if someone would invest that much time and effort into Mint (or any open source project of mine) I would welcome it and or at least start a dialog on how we could integrate their work into the language.
Anyway, I thought I share my Elm story with you (although probably a bit late :))
Cheers
[1] https://github.com/gdotdesign/elm-ui
[2] https://github.com/gdotdesign/elm-github-install
[3] http://elm-directory.herokuapp.com/ https://github.com/gdotdesign/elm-directory
[4] https://github.com/gdotdesign?tab=repositories&q=elm-&type=s...
[5] https://www.mint-lang.com/ https://github.com/mint-lang/mint