"how are the discoverability and accessibility equations going to be solved?" - this is the whole challenge.

I referred to the WorksonARM project in the blog: https://www.worksonarm.com/

WorksOnARM solved this problem for ARM by making machines available through Equinix Metal's API for development and testing.

Hardware manufacturers can ship boxes to developers, give them access through an API like Equinix Metal's, or some other approach, but one way or another developers are going to need access.