Even if the quality has gone down, it's still better than the other mobile OS's out there. That's not to say Apple should rest on their laurels.
But to address the things you mentioned, I don't see how YouTube not being there affects quality. It's just not a native app anymore, and if anything, doing that has allowed for a BETTER YouTube app to show up, in the app store, for free. Don't see how that's a problem.
You seem to be complaining more about specific apps, not quality overall. The maps app is, for the better part, a disaster, you're right. I find it weird that they allowed for it to be released in that way... I can understand, partially, because of them wanting to compete with Google and so on, but it's still a bad move. Ofc they can improve on it with time, but I think most of us trusted that they'd do a better job.
Other than that, I can't think of other major problems. Every software updates, iOS, OS X, they all come with a few bugs here and there... It's just normal, and personally I haven't been having any App Store problems.
But to sum it up I guess, I wouldn't say the quality of iOS has gone down, in fact, it's gotten better. It's just a specific app that has turned to ****, but in other aspects, things have improved. The weather app is much more polished and beautiful, the stores and music app (I guess this is a matter of opinion) look much more modern and beautiful, etc. Small changes, like the little metallic circle in the volume slider having that reflection effect changing depending on how you hold your device, shows their attention to detail, and small details like that affect quality a lot.