If you want to see an example of the fragile base class problem in action, just run one of the, I dunno, 20 or so Cocoa applications I wrote in Xcode (from 2007-present) on Sierra. I spent a lot of time ensuring they were bug-free on 10.6-10.10. Here's a hint: new design idiom, new base classes. Don't be trying to staple new underwear onto the baby.
For a comparand, I have 16-bit Windows applications I wrote in 1989 that still run in the WOW layer. This is why Microsoft rules the corporate desktop and no amount of clever advertising (throwing shade) is going to change that. Sadly.
But I guess you win some and you lose some: yesterday I learned that iMovie '11 runs perfectly on Sierra. For certain jobs, it's my tool of choice (4:3 small and fast m4v files).