Many (most?) software engineers (most?) cringe at the state of the npm ecosystem—even if they hold their nose and participate in it. I definitely wouldn’t consider that an improvement to the state of game dev software engineering practices.
Do you dislike cargo as well or just npm for some reason?
I've worked in languages that had large standard libraries where it was common to only include a few large commercial external dependencies, and I've worked with javascript and ruby on the other end of the spectrum.
I don't think one style has a clear productivity advantage over the other. Other than maybe at the very lowest levels of beginning software engineers (even then I'm unsure because of the decision paralysis common for beginners in these ecosystems).
As to the original point, however, Unity and other engines already have package managers and asset/code stores. What you're essentially asking for is for Unity to remove many of the features they already include and pull them out into packages. Now you're running up against many of the organizational issues I've already talked about. Not saying it's impossible, in my experience it's just not likely to work out long term.