So in that spirit, if the question is "why does it seem like OS X isn't of the quality it used to be?", the simplest answer might be that Apple just doesn't care about OS X anymore.
I mean, look at their financials (http://files.shareholder.com/downloads/AAPL/3797471556x0x789...) On page 29 they list their net sales for all their major products from 2012 through 2014. And the things that jump out from that page are:
1) The iPhone business generates 4x as much revenue for Apple as Macs do -- and if you add in iPad sales that figure goes up to 5x; and
2) The iPhone business has grown by more than 10% each year, while Mac sales have been essentially flat.
So if you're Apple, where does the Mac fit in your product line these days, exactly? People still buy them, so it's not like you completely don't care about them, but the importance of those sales to your bottom line is diminishing year by year as sales of mobile devices dwarf sales of Macs. And there's no indication that those trends are going to turn around anytime soon -- no new major piece of hardware coming from Intel or somewhere that's likely to light a fire under sales of conventional PCs, or software package so compelling it would drive the Mac into new vertical markets.
All of which means that maybe, if you're Apple, you see the Mac business as basically a cow to be milked at the lowest cost possible until it dies of natural causes. You don't cut the hardware build quality (at least, not yet), but you don't put a whole lot of effort into developing revolutionary new features for OS X or wringing every last bug out of new releases of OS X, either. You just do the minimum possible to keep those existing Mac customers from jumping ship, or at least to delay the moment when they do jump ship for as long as possible. And by the time that day eventually comes, you won't care because Macs will be a footnote to your real business, your device business.