I think a key here is hypermedia. APIs can only be machine-discoverable if there are hypermedia links pointing to them (which is how this helps). If there are no links, then some human must hardcode it.
This also is a reason for including hypermedia in your API, because really, being machine-discoverable is not just something that the API itself benefits from... each resource and state can benefit from included hypermedia as well.
I agree that hypermedia is a more optimal solution for API discovery. APIs.io + APIs.json + machine readable formats like Swagger will provide us with a bridge between what we have, and what we should have (aka hypermedia). We'd all love to have a perfect reality, but unfortunately we get the one we have. ;-)
This also is a reason for including hypermedia in your API, because really, being machine-discoverable is not just something that the API itself benefits from... each resource and state can benefit from included hypermedia as well.