Hi Chris,
Thanks for bringing this up this issue. It is a concern and we have given it a bit of thought. The good news is that mapbox-gl 1.8 is working pretty well right now, so there isn't any pressing need to move to a new package. However, we are keeping an eye on alternatives in case we do need to migrate to another solution in the future. Our main concern is that any mapbox alternative supports the mapbox-gl style spec. This will help ensure the switch is minimally disruptive to core Arches as well as any custom work that project developers have done for their own implementations. MapLibre would, of course, meet that requirement, so it seems like a very good candidate.
When the time approaches that we may need to change mapping libraries, we will definitely announce it ahead of time and we'll be looking for input from the community. In the meantime if people learn about projects like MapLibre that seem like good solutions, we'd love to hear about them.
Thanks,
Cyrus