This sounds nice on paper. There's just one problem: you mention one overarching framework. That's the game engine. And no matter how you slice it, it's going to be a beast.
I also think you're over-estimating how much of a "beast" you need as a minimal structure for building a game around. I have built a lot of hobby games, and if you're not trying to make an AAA game, you can get really far with a simple game loop, and a few single-header libraries for things like audio and asset loading.
That would be one of the advantages of a more modular approach: you could choose a right-size solution rather than working from the assumption that you need this hugely complex monolith to build off of, and potentially not needing a lot of that complexity.
It’s an organization/people problem. Eventually if your tiny hobby game engine gets big enough people start trying to use it to make the equivalent of AAA games. Then either those users (or your investors if you’ve gone that route like Unity did), start pushing you to support those edge cases.
Eventually your elegant modular code is dwarfed by the all the edge case handling that got crammed into the framework—either because it was so performance critical that it had to live there or (the more likely case) because it was much faster to hamjam it in than spend time thinking about how best to architect it.
There’s no reason the vast majority of time that you need to pay the overhead of using network calls to enforce your boundaries. Yet time an again you see companies willing to pay the microservices tax because it’s just so hard organizationally to enforce modular boundaries over time.
Then we have the scalability, then we have composability, then we have different security models, then we have code isolation.
There are many advantages to a microservice model. But that doesn't mean the microservice model is good for a game, unless it is backend code.
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.