If you look at the macOS feature history, it's pretty clear that the bulk of the team got shifted to iPhone in 2007 and never really recovered. The widely acknowledged Snow Leopard high water mark happened shortly after.

To be fair, Apple can still pull off the occasional amazing feat of vertical integration -- HDR, APFS, keeping audio latency under control despite the relentless assault of apathy from all directions -- but they never had the same level of consistent drive forward, at least not until a year or two ago when the big push for AI integration started. Apple gets ragged on here, but I think their integration is actually some of the best. They were putting neural cores in chips back when that sort of thing got mocked, not lauded, and every step has been thoughtfully tied in rather than airdropped from a ChatGPT science fair project. But they never got good at building or deploying leading-edge models themselves; I hope they turn it around because this is important.