I can't comment on the performance of 8.4 on A7 devices, but I will say this. 8.3 was kind of jittery on my iPhone 6... after playing with iOS 9 for a while, I decided to go back to 8.3 by doing a clean install as new, and I haven't experienced performance like this since the iPhone 5 on iOS 6. 8.4 runs just as well, too. I did this on Monday, and there are only 18 entries in my Diagnostics and Usage log- I was getting dozens of entries per day on 8.3 prior to updating my phone to iOS 9. I'm no expert, but I think the earlier versions of iOS 8 were so awful that their negative effects persist until you set up a device as new on at least 8.3 or 8.4. iOS 9 is much more promising than iOS 8 ever was, so I think Apple learned their lesson on releasing half-polished software.
Now, regarding the music app, I think it is a much better app in 8.4. That is of course because I'm actually using Apple Music. The ability to ask Siri to play any song in the Apple Music library while I'm driving is a game changer for me. I imagine that someone who doesn't plan on Apple Music (and especially someone who uses home sharing) would have mixed feelings about 8.4 at best, and I understand that. But my point is, it is a vastly more useful app for many people, so you really can't say that it is objectively a worse app than it used to be.
Apple doesn't make it any harder to switch to Android than Google makes it harder to switch to iOS. As long as you have your iTunes library, it's a trivial task to move whatever songs you want onto an Android device. If you use Apple Music, that's coming out for Android soon. If you use any other streaming service, you're good to go right now. No matter which platform you're on, you have to pay for apps all over again. Neither Apple or Google make it artificially more difficult to switch platforms, it's just the nature of the beast.
And if you keep buying Apple products despite your concerns, you're letting Apple know that you're accepting their dick moves. Apple devices do everything I want them to do, so don't rely on me to rally for your cause. I understand and sympathize with your concerns, but you need to be the one to vote with your wallet, just as I would vote with mine if Apple took away significant features that I depend on.
iOS 8.4 is still the same jittery mess on the 5 iPad Airs I've installed it on so far, albeit a bit better. My Mini 2 has more predicable jitter than my iPad 2 which is a worry.
It is a worse app, because a better app would have included all the features of the old one. Its pretty simple. Apple has a lot of money so they can afford to do that.
It is extremely difficult to switch off the Apple ecosystem. Can I copy my data from my iPhone to an Android phone? Not so much. I've used Macs my whole life, and while I do also use Windows and a Windows tablet, my iPhone and iPad are my primary devices. Until you can use iCloud + iMessage on an Android device it is very difficult. Put simply, Apple is still better than the rest, I just don't see why it needs to continually pull moves like this.
My way of expressing my concerns is through filling out feedback for Apple. Honestly if nobody complained, Apple would get away with anything and everything.
My point is people will and should complain. Its how things get fixed.