Apple Music was introduced at WWDC 2015 as the “one more thing” on June 8th. It launched 3 weeks later on June 30th with 8.4. Obviously each major release comes out 3 months after it’s announced at WWDC, but you are trying to compare the iOS 11 feature delay with Apple Music’s 3 week launch after announcement timeframe. Not comparable.If I recall correctly, Apple Music was introduced at the March Event. Thus it would be 3 months. Apple Music was part of iOS 8.4 and that went into beta on April 13, 2015 and the GM shipped June 30th.
And it doesnt matter if it was introduced at WWDC 2017, because iOS doesnt even ship until 3 months later.