Are you totally and completely unaware that Apple actually does dedicate some annual cycles to bug fixes, performance improvements, and stability? And that they've done this many times in the last 20 years?
So what does the software look like up until those 'annual cycles'?
It's pretty clear from the economic incentives of the companies producing the software, the sheer quantity of exploits found, and the surprisingly low amount of QA that would be needed to catch most bugs that the aforementioned view is correct.
Your generalization seems a bit broad.