The decline seems real to me. A shortlist of things I've noticed:
* Spotlight no longer finds things as easily. I used to use it for everything. Since updating to El Capitan, it has missed some exact match folders. Planning to switch to Alfred.
* iWork was gutted in '13. People used to use Pages professionally. I'm now using Pages '09, and planning to transition to Word or Latex. I tested Pages '13 intensively, and it fails for even basic publishing.
* Siri can only work with default Apple apps. And those default apps are getting worse. So Siri takes a hit with every app that declines. I used to use Mail, now I don't.
* Constant Wifi issues. I frequently have to turn off wifi, then turn on. On my home network. This never happened pre Mavericks.
* In general, all my Apple default software on my iphone is sitting in a folder titled "apple", which I never use. I don't think I use any Apple default App.
* I avoid icloud. It sends scary "do you want to delete all these files" messages if you ever unsync a device, and it's not clear which actions produce which effects. iTunes has a history of destroying files on syncs, so I can't trust iCloud. Even now, itunes will add apps to my device if they're in my library but I deleted them from my phone. It does this without asking! Any other cloud app has figured out how to handle deletions from one device.
Pages 09 hit the hardest. It was wonderful software. I used it for print publishing, and it just worked. Easy to use, incredibly powerful. Have a look at their manual for the level of care they put into their software, as recently as 2009.
Pages 13 can't do half of that. Very basic stuff like "facing pages" for books has been left out.
https://manuals.info.apple.com/MANUALS/0/MA663/en_US/Pages09...
Edit: A comment below pointed out that, I do in fact use default apps. I had taken them for granted. These ones work well and I use them:
Messages, Phone, camera, photos, clock, wallet, calendar, music (UI got worse on this one). Reminders I use occasionally because of the Siri integration.
There are some issues with some of them, but mostly they work pretty well.
On the mac, the only default apps I use frequently are textedit and Preview. Previews remains excellent. I use spotlight, but as noted above it got worse.