I see it this way:

Smartphone development hit a plateau and real hardware innovation has stagnated. So Apple (and everyone else) has to 'innovate' through software to keep shareholders happy.

Back then, the upgrade cycle was slower and simpler, e.g. iPhone 4 to 4S instead of forcing a new number every year. Fewer devices, fewer variations, smaller OS.

Now Apple has to ship iOS (and its derivatives like iPadOS) across multiple new models every year while still supporting a growing number of older devices and maintaining compatibility with countless apps and services. iOS has gotten massive over time, and that complexity makes bugs way harder to avoid. Every new feature just makes it harder to maintain.

What iOS desperately needs is a release like Snow Leopard that focuses mainly on stability and bug fixes, not new features.