WWDC is for developers. So they can get help directly from Apple, and hear about what’s coming up, so they can start to contemplate how Apple’s planned features might interact, inform and inspire their own future features/products. It’s not the start of 3-3.5 months of work to get those features ready; they’ve been many months or even years in the making.
Apple will often say feature xx is planned for a later version of OS Y.0. They didn’t promise Apple Cash, Messages in the Cloud, AirPlay 2 or whatever other new feature someone wants—and they want it now, daddy—would be in the .0 release. Quite the contrary.
Features are ready when they’re ready. The alternative to needing some extra weeks or months to implement some feature isn’t holding that feature for 6, 8 or 10 months until next September’s .0 release. That would be crazy, simply incredibly disruptive. It’s not a viable way to schedule software feature rollouts.
Which is not to say that some features might not be delayed until next year’s .0 (or later). For every release cycle, there’s always the above the (cutoff) line and the below the line features from the must-do list. Everyone goes like hell to get the most important features in a releasable state. (Which of course doesn’t mean bug-free, but you do everything you can to make sure it’s showstopper-free... then you cross your fingers and pray.)