From Steven Troughton-Smith’s fascinating (and successful) attempt to write a Mac app capable of running from System 1.0 all the way up to Yosemite:

The more I dug into it, the more I came to the conclusion that Carbon was probably one of the most important things Apple did in building OS X. Even today it provides source compatibility for a huge chunk of the classic Mac OS software base. It kept the big companies from ditching Apple outright when they were needed the most, and gave them a huge runway - 16 years to port perhaps millions of lines of code to OS X while still being able to iterate and improve without spending thousands of man-years upfront starting from scratch. Over time, of course, Carbon has improved a lot and you can mix/match Carbon & Cocoa views/code to the point where you can’t realistically tell which is which. I appreciate what a monumental effort Carbon was, from a technical standpoint. That Cocoa apps always felt ‘better’ is more to Cocoa’s credit than Carbon being a bad thing - it’s a lot easier to see that in hindsight.

Part of me wonders if, in 2039, someone will try to write an app that runs on iPhone OS 2 and whatever version Apple’s mobile (if “mobile” will still be a concept) OS will have reached. Let’s check back in two decades (hopefully?).