It’s far from a complete solution, but to mitigate this specific avenue of supply chain compromise, couldn’t Github/npm issue single-purpose physical hardware tokens and allow projects (or even mandate, for the most popular ones) maintainers use these hardware tokens as a form of 2FA?

code becomes trusted by review, but these crowd sourcing efforts to do so fizzled out, so in practice we have weak proxies like number of downloads

the implicit trust we have in maintainers is easily faked as we see

All maintainers need to do is code signing. This is a solved problem but the NPM team has been actively rejecting optional signing support for over a decade now. Even so maintainers could sign their commits anyway, but most are too lazy to spend a few minutes to prevent themselves from being impersonated.