What mature products deteriorated (apart from iTunes) and in what concrete ways?

Mail is much better than it was even a year ago, XCode too, the OS is stable...

And FCPX didn't deteriorate over 7 -- it was a written-from-scratch reboot of the platform that just happened to cut some features people used (most got back with a vengeance).

There a number of examples in the surrounding threads. I find most software getting worse these days on all platforms. See CADT: https://www.jwz.org/doc/cadt.html

And most of that is simply looking at the past with rose tinted glasses.