I'm still hoping that the OS will make our lives (programmers and users) easier, rather than us having to know about the low-level stuff, even if (these days) we usually handle it by invoking yet another library.
despite the needlessly hostile tone of my initial message, i'm curious what you see as the pros and cons of "yet another library" knowing about the low-level stuff rather than the kernel knowing about it
Size: The more externals you drag in, the bigger and less efficient the apps will get. An extreme version of this are the deployment packages like flatpak and snap - you drag it a substantial copy of an OS! Bigger needs more storage, slower startup, etc.
Difference: Every library does things a little differently - uses different init schemes, different serialisation, different naming conventions, etc. Makes for steeper learning curves (for users and programmers), incompatabilties, etc.
Knowledge: over a few projects, you'll need fairly detailed understanding of many libraries. Makes sense to consolidate this into one place (the OS). It will be less than perfect for some apps, but realistically, so are the libraries.
Bugs: Really a by-product of the above - but bigger systems (more libraries and/or more hand-cut low level code) invariably have more bugs.
I'm afraid the only pros that comes to mind:
Potential performance: low level stuff gives you the option to fine tune. But: #1 for almost all apps, a better design gives performance more easily than low-level coding, #2 the industry default is to accept poor performance and buy a faster machine ;)
Familiarity: using a new paradigm involves changing thinking and skills - uncomfortable and time consuming.
these all seem like arguments for standardizing on a particular implementation of any given functionality, rather than having many different implementations
they don't seem to bear on the question of whether to put that functionality in userspace or in the kernel
you are presumably aware that at times these advantages of standardization on a single implementation are outweighed by its disadvantages; see http://akkartik.name/freewheeling for some interesting exploration of the opposite extreme from the kind of high modernism that seems to be your ideal
