I find this application/library distinction hard to reconsile with the many long-lived, stable applications that exist. AutoCAD, for example, is older than I am.
Lets take Autocad as a long-running example. Its first release was 1982, and had an update this year, in 2020. What's a good example of "application" vs "library" code here?
Lets take the user interface, which is almost always "application" style, with short lifespans. Back in 1982, you probably had to write a custom serial-communicator to interface with the mouse (or trackball). Eventually, Windows is released and a standard mouse-interface becomes common practice.
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A few decades later, the 3-button mouse with scrollwheel (with the wheel being the 3rd button) becomes popular. Adding new features to the application, and the overall design of the UI would have to change for modern sensibilities.
Then Ribbon happens. Hate it or love it, Windows programs are now Ribbon-based. Gotta overhaul the UI AGAIN to match the changing times.
This progression from 2-button mouse -> Windows driven mouse -> 3-button mouse with scrollwheel -> Ribbon -> touch-enabled (??) is "Application code", requirements that change with the times. Every few years, the code interfacing with the user was largely rewritten, to match the (then modern) expectations of its userbase.
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Of course, there's the "library chunk", which probably solves geometric constraints or something like that. That part may have never changed throughout the life of AutoCAD.
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Application code CHANGES. That's the important bit. You cannot expect applications to look the same over decades. I'd be very surprised if AutoCAD still had their DOS GUI laying around (https://www.scan2cad.com/wp-content/uploads/2016/06/autocad_...). That sort of code is thrown away when it is no longer fashionable.