Around 10.9, MacOS reached a "feature complete" state. Now things are just being changed for change's sake. Running older Android versions reminds me of how fast and slick they were, and how many features have since have just added complexity in meaningless ways (they've changed the "silent" mode roughly 4 times since 2011?) and introduced "features" nobody asked for. Half the features end up getting removed later.
As an example of good software that doesn't perpetually introduce bugs, take venerable *nix Desktop Environment XFCE. (Screenshots here https://xfce.org/about/screenshots) It has a release every few years and doesn't add anything major or change in breaking ways - it works, and works the same as it always has for decades.
(edit) I had another thought - since Apple the company is now so large and most of their resources are focused on churning out new products year after year, it could be that they've stretched themselves too thin and the people that could uphold the quality standard of the systems either a) aren't as much of a hardass as Jobs was and don't have as high standards, or b) just don't use the products on a regular basis. I bet you there's someone working at Apple right now getting paid >$250k/yr that hasn't touched macOS in over a year.