I don't think they are holding off for IOS 9 otherwise they would not have redesigned the music app until IOS9.
What makes you think that 8.4 will be released on the same day as the WWDC keynote?I don't know, just seems weird that they would introduce it in 8.4. Especially when they haven't tested it at all in the 8.4 betas.
I never said I thought that, I have no clue when 8.4 is coming out. I honestly thought we would see GM today, now I have no clue.What makes you think that 8.4 will be released on the same day as the WWDC keynote?
If they need to add stuff related to the streaming music service they could easily run more beta cycles including The additional features that they don't want anyone to see yet.
Alternatively the service could be all back end... Paying for the service may just allow you access to download vast amounts of stuff from the iTunes Store... So there may not be anything to add to the music app.
This is how it will be:
iOS 8.4 will be released at WWDC for the public (without the new music service). Then when iOS 9 launches in September there will be the new music service.
For OSX almost the same 10.10.4 released at WWDC but with nothing new. Than 10.11? will be introduced at WWDC and released at the keynote alongside .....