Cool, thanks! So yeah, these decisions were made during the initial implementation, and most of the discussion has only been on one part of this, and so I would say it's an under-explored area. Here's what I've got for you:
1. the only part of this that's been in recent discussions is using the URL as a namespace. This has led to questions around how those are represented in code, given that URLs are not identifiers. Also, using DNS for namespaces is mutable and costs money, this is a huge barrier to adoption that we did not want. Publishing should be free and easy.
2. I think you've already identified some of the tradeoffs that made this be rejected: for reproducible build reasons, referring to external services is considered unacceptable. This is due to stuff like downtime of services we don't control, but also the mutability of those services. In theory, you could proxy them, but it's not as clear what this actually buys you, because to get those advantages back, you'd want to always refer to the proxy, and now you've re-centralized everything.
Another way to look at this is that all of the solutions you've proposed are significantly more complicated than what we've built. That complexity is for good reasons, but given 2, it's not clear that they're good enough to justify the costs. The current situation is significantly simpler for all parties, and the end experience ends up the same anyway.
I hope that helps!