Much of the system can be complete like this with forethought. The pieces that cannot can be factored out to the edges.
The majority of devs generally are happy to tack on their features and PRs to whatever random scaffolding they can, without regard or awareness for how their individual component fits into the larger system, or how it may be extended. And to be honest it’s not necessarily a bad thing, because they do need to get work done, and merging PRs shouldn’t be reserved for the enlightened.
I guess I’m just pessimistic. The reason we don’t see perfect software is because we are not capable of producing it. At a certain point it all becomes spaghetti. If you work with software that isn’t spaghetti, it’s only because the people who care about it not becoming spaghetti haven’t left yet. This is good, but eventually they will leave, standards will decline, and you will become one with the pasta.
Forethought is only possible if people tell you the requirements precisely correctly upfront. Real systems design is you get 90% built and someone drops a hard requirement that's also a layering violation on you.
Math arises from first principles, human behavior does not.