The constantly changing frameworks and languages are a useful strategy from platform vendors - they do help provide platform lockin, and make it very very difficult to provide cross platform apps. That is a real threat to Apple with offerings like flutter from Google. Of course there are other reasons to make a clean break with the past, but there is a lot of work just to stand still in developing for a platform like Mac OS or IOS - they don't highly value backwards compatibility and often introduce sweeping changes. Constant change is in the platform vendor's financial interest, and I don't think it's unfair to point that out.
Just to take one example - apps built for the original iOS are now almost entirely obsolete, and it's not worth reusing any of their code in a modern swift app.
QuickDraw and Quickdraw GX aren't UI frameworks (like PowerPlant, AppKit or UIKit), and they aren't a widget library (like HIToolbox). They're much more lower-level than that; drawing libraries on the level of Quartz, Cairo, Skia, or GDI.
Ooh, that mention of PowerPlant that takes me back, I remember CodeWarrior too back in the day.
Yes you're right there are various levels here - sometimes it's hard to distinguish them. There have definitely been at least 5 UI toolkits though, and probably more, though of course over the life of Mac OS that is not terribly unexpected. For those who lived through the transition to OS X, this sort of churn is not unusual, and I do think it does benefit platform vendors - they have zero incentive to keep supporting their technologies over decades and every incentive to increase churn.
Quickdraw GX I remember particularly because there was such fan-fair about it as a replacement for all your graphical needs (such as drawing text), and yet it was dropped before it could even really be used (same with Quickdraw 3D). I can't remember what apple called their UI toolkit at the time, which was based on Quickdraw, then GX, but I don't think it was Carbon, that came later didn't it? I think I've found it now, was it MacApp?
I firmly don't believe they're changing frameworks to "provide platform lockin".
Development philosophies change over time, like the other's said, some of those UI frameworks you mentioned are 20+ years old.
It does suck that we can't use it on other platforms, and I'm sure they don't want to spend resources on doing so, but there's no way the main motivation is to keep people switching frameworks constantly.
That's certainly not why the programmers involved do it, they do it to improve things, but the effect is platform lockin, which is not at all unwelcome for platform vendors and I suspect is why the companies are quite happy with constant churn in languages and tools. I'm not suggesting that is 'why they do it' just that it is a strong incentive to keep doing it.
If they don't control the language and tools for the platform, they don't control the platform.
