I can’t speak much for the rest, but old Mac codebases in particular hold up better than you might think. You can write an application that runs on every version of Mac OS from 1.0 (circa 1985) up to High Sierra with just a handful of conditionals through the use of Carbon [0]. Porting pre-OS X software should be similarly simple, given that it’s written in C or C++ (Pascal was somewhat popular for classic Mac apps, not sure what getting that code running on modern machines would look like).
When it comes to “new world” Mac stuff, the things keeping something written for OS X 10.0 from running on modern macOS have more to do with API deprecations than anything else. Objective-C is still widely used and is well supported (even with Swift’s growing popularity), so usually getting an old codebase of this nature running just means fixing a couple of deprecation errors, fixing a bunch of warnings, and hitting compile. The nature of change in the native sphere has largely been evolutionary, not revolutionary, so it’s largely stayed the same with new things being added. It’s not like the web where wheel reinvention and fads are a constant.
When it comes to “new world” Mac stuff, the things keeping something written for OS X 10.0 from running on modern macOS have more to do with API deprecations than anything else. Objective-C is still widely used and is well supported (even with Swift’s growing popularity), so usually getting an old codebase of this nature running just means fixing a couple of deprecation errors, fixing a bunch of warnings, and hitting compile. The nature of change in the native sphere has largely been evolutionary, not revolutionary, so it’s largely stayed the same with new things being added. It’s not like the web where wheel reinvention and fads are a constant.
[0] http://blog.steventroughtonsmith.com/post/109040361205/mpw-c...