1) when a new OS comes out, some apps designed for the old OS have problems. This works out over time as most apps are updated. Some old unsupported apps are left in the dust and no longer work under new OS versions. this has been true since DOS.
That's a weird example given that Microsoft have always been religious about backwards compatibility and app-compat testing, to the point where you can in fact run apps written for DOS in Windows and have them work 20 years later.
Apple, on the other hand, have managed to ship software updates that break Chrome in recent times - that's not exactly some obscure hard to test for app. Yosemite was/is riddled with insanely obvious bugs that show me Apple just doesn't care or make time for OS upgrade testing. It's not some fundamental thing.
For example after the upgrade the IntelliJ UI is flickering and broken for no obvious reason, presumably something related to their new transparency everywhere. If you disabled said transparency in the accessibility prefs window, beyond being incredibly ugly, the corners of rounded popover windows like the volume indicator went black! They weren't even doing transparency blending right! Come on guys, whoever implemented the "no transparency" feature must have realised it was unfinished, but they shipped it anyway (fixed in 10.10.1 I think). Yosemite also trashed support for my USB smart card stick for, like, 4 months. The manufacturers were eventually able to make it work again, but it now comes with a list of stupid glitches that are due to new bugs in Apple's libraries - one of them is literally "if you use app X Y or Z, then apps A and B won't work until you restart your Mac". WTF? Apple didn't even make any upgrades in smartcard support in recent times. How did they manage to break it so much?
Yosemite is full of crap like this. I'm ignoring the subjective things like I think it looks worse. Yet, I cannot identify a single feature or improvement that I've benefited from post-upgrade. I had to do it to test the software I'm writing and keep up with security updates, but if I'd known and had free choice, I'd have stayed on 10.9
But this has been a trend ever since iOS. As was pointed out elsewhere on this thread, it's an open secret that Apple basically have one team of their best people that Jobs kept moving around between the most important projects. OS X has suffered for years because of this.