how they solve the core maintenance problem?
It very much depends on the personality and social skills of the language leaders and is also tied to the funding model of the language, the two being intertwined together.
You can see a discussion between Evan (Elm), Jeff Bezanson (Julia) and José Valim (Elixir) about funding a language here on Jean Yang’s channel: https://youtu.be/f0ouXATi25I
As hinted there, what works for some is different than what works for others. Collaboration at the scale of a widely used programming language requires a lot of communication, even just for maintenance and bug fixes. And as Evan says it himself in the video, it is not where his strong suits are. Which I’d say are rather around overall language design and technical achievements. So it’s understandable that he wants to spend his time there instead.
Back on the process regarding possible improvements. I don’t think we should restrain ourselves to not do anything, just by fear of community fragmentation. If what we try to achieve tries to stay as close as possible to elm but improves user experience (like @rupert approach), this will ultimately be beneficial and bring new people instead of fragment the current community.
That being said, all these initiatives are individuals from the community spending some of their free time. So “Elm is gonna stay as it is now for quite some time” is a fair assessment of the situation in my opinion because progress of individuals on their free time is something measured in months rather than weeks. At least as long there isn’t a company with the will and financial capability to support employees on the matter.