Obviously Elm is his passion project, that's fine. But nobody who locks down their compiler on a supposedly "open source" project can actually seriously expect others to engage with that in good faith.
Furthermore, the absolute state of just trying to modify Elm to be more fit for purpose is horrendous. Every time I see a parallel implementation or a fork, the author has to bend over backward with platitudes to placate the Elm masses and ensure them that this project isn't trying to "split the community," which is a nonsense idea to start with imo.
It feels cultish, on top of a project that is actively hostile to people who want to use it any way other than how the BDFL has mandated its usage. If you want to say the problem is with the users, that's fine, but that's exactly the attitude keeping me from ever engaging. I personally think the problem is with the people who want such fine control over their technology and community, as opposed to understanding that they can really only shepherd their implementation. That's about as far as their reach rightfully extends.
EDIT: also, the fact that they aggressively lock down their compiler and have such hostility to anyone working in parallel is imo the definition of "holding it back"