[HillRat]

Sign the package with hard keys and signature.

That's really the core issue. Developer-signed packages (npm's current attack model is "Eve doing a man-in-the-middle attack between npm and you," which is not exactly the most common threat here) and a transparent key registry should be minimal kit for any package manager, even though all, or at least practically all, the ecosystems are bereft of that. Hardening API surfaces with additional MFA isn't enough; you have to divorce "API authentication" from "cryptographic authentication" so that compromising one doesn't affect the other.

[Hackbraten]

How are users supposed to build and maintain a trust store?

In a hypothetical scenario where npm supports signed packages, let's say the user is in the middle of installing the latest signed left-pad. Suddenly, npm prints a warning that says the identity used to sign the package is not in the user's local database of trusted identities.

What exactly is the user supposed to do in response to this warning?

[biggusdickus69]

This is a solved problem. https://en.wikipedia.org/wiki/Web_of_trust