They certainly don't seem to have much time to spend between releases to concentrate on getting the OS itself as efficient as possible, nor do they seem to have any reason to do so.
Patrick, I don't know how Apple manages software development, but every company I worked for in my IT career from the 1970's until I retired 10 years ago had a "leapfrog" development. One team would be working out the kinks in the next release, identifying bugs, fixing them, or at least documenting them for later repair if the release was close. Sometimes, they had to remove pieces/parts of the pending release because they just couldn't be fixed on time. That was the "next release" team.
Another team would be working on the design of the release after that one, getting the updated code from the first team as it was in work and making changes to it for whatever features/functions that were to be added in that following release. In addition, they got handed anything that had been cut from the upcoming release to be added to their work.
Once the software was released, the first team started work on the third iteration, with the second team now working on the next release, with the planned release date as their deadline.
Lather, Rinse, Repeat. The life of a programming team in a large company.
I wouldn't be surprised if Apple didn't have even more teams, working 3-4 releases away on the hardware in the laboratory stage. As a hardware company, they are eternally in the build it/try it mode to see what comes next. So, somewhere in Apple is a team working with M3 or M4 (or even maybe M5) chips, with a version of the OS that may someday become version 16, or 17. If they don't, they are falling behind.
And no company works to get ALL bugs out or ALL features they thought of included. At some point the software has to be released. If you try to go for ALL, you get NONE because no non-trivial software was EVER perfect. You get to a state where it is "acceptable" and you release it. And keep working on it.