This criticism is spot on; it is exactly what I observed in late 2018, early 2019 as well. It's an awesome language, but there is basically a big case of 'vendor lock-in'. Certain compiler features are only available to the maintainers, nor is there any way to use alternative package sites.

I don't want to know what would ever happen if the Elm developers decide to take https://package.elm-lang.org offline. That will leave so many projects out there in an unbuildable state. Even if you saved source tarballs of the core packages, there is no way to tie it into the build yourself. The Elm compiler must load it through the official package site.

I would thus personally strongly advise against using something like Elm to build any piece of software that needs to be maintained over time. It's too much of a risk.