I think this is a really big issue, and I hope people are paying attention. We spent a lot of time during the second review of this proposal discussing the fact that the design of this service means that it is heavily biased toward GitHub. The only thing that somewhat mitigates that substantial issue in the design is the ability to create a proxying registry that can operate to confederate multiple other registries. I'm not completely happy with that option, but I'll take is as a compromise.
It is critical that there is an ability to create low-bandwidth package registries that can confederate packages from other registries. Mandated support for redirects creates this opportunity.
The ability to redirect to a source registry substantially eliminates issues of high-bandwidth usage, storage cost, cache coherency, and high latency for confederating proxy registries.