Mac OS X wasn't originally planned to have Carbon. Adobe basically told them there is no way they are completely re-writing all their apps for OS X. So Apple took another year, added Carbon, and Adobe ported Photoshop over.
Apple doesn't benefit from higher average quality of developers; they just benefited from having much fewer of them to support. The long tail of Windows is really really long -- businesses run on multi-million dollar software packages designed in the 90's. Apple has no such ecosystem.
The big ecosystem they do have (iOS apps) is very different and far more forgiving. There are thousands of unmaintained iOS apps that simply don't work anymore but they only cost $1.
While I agree with your premise (that Apple's lack of care or rigour for platforms likely stems mostly from having very few people to support), the specific story being provided for Carbon doesn't fit the history to me given that not only was iTunes written with Carbon (and was a holdout in switching to AppKit for a lot longer than Adobe), but so was Finder (a core piece of OS X, and that didn't transition until 10.6).
It is true that iTunes and Finder were written in Carbon, but it is also true that Apple's original plan was for Yellow Box (the OpenStep API that later became Carbon) to be the main API for developing programs under Rhapsody, which was essentially NeXT with Mac OS 8 theming and UI guidelines. Rumor has it that Adobe and Microsoft balked at the prospect of having to port their applications to Yellow Box (there was no guarantee in 1997 that Apple's purchase of NeXT would pay off), which led to Apple announcing its Mac OS X strategy during WWDC 1998, which included Carbon as a first-class API for the development of Mac OS X applications alongside Cocoa (Yellow Box).
Regarding iTunes, one detail about Carbon that is sometimes forgotten is Carbon originally ran on Mac OS 8 and 9 to facilitate the transition to Mac OS X (https://en.wikipedia.org/wiki/Carbon_(API) ; to quote, "Carbon was introduced in incomplete form in 2000, as a shared library backward-compatible with 1997's Mac OS 8.1. This version allowed developers to port their code to Carbon without losing the ability for those programs to run on existing Mac OS machines.") Given that Mac OS 9 was still current when the first version of iTunes was released, it made sense for iTunes to be developed as a Carbon application (since Carbon applications can run on Mac OS 8 and 9 as well as Mac OS X) rather than as a Cocoa application (which can only run on Mac OS X).
I remember reading an article on Rhapsody. I was then out of touch with the Mac world for a year or two, and I later started hearing about Mac OS X. The Adobe / MS story is well-known, but how was Mac OS X different from Rhapsody? It seems that Cocoa is a rebranded Yellow Box, and Carbon is a rebranded Blue Box. What was actually different?
I encountered a few unmaintained apps -- again the long tail is long. Some failed in more and more ways with each new iOS version. Forcing developers to update their apps is exactly the issue, really.
Apple doesn't benefit from higher average quality of developers; they just benefited from having much fewer of them to support. The long tail of Windows is really really long -- businesses run on multi-million dollar software packages designed in the 90's. Apple has no such ecosystem.
The big ecosystem they do have (iOS apps) is very different and far more forgiving. There are thousands of unmaintained iOS apps that simply don't work anymore but they only cost $1.