Mail.app and Calendar.app are apps that I've stopped using completely in favour of third-party apps.
Then came the Apple Watch. With the Apple Watch, these first-party apps become much more appealing. Calendar events on the watchface are simply better than calendar events anywhere else and using Calendar.app is the only way you can have them.
It's safe to say that Apple will eventually give developers the power to create integrations that work as well as their own. There will probably even be WatchKit Extension Complication Extensions that let us create our own complications on watch faces.
I'm sure third-party apps will get onboard and implement it all. But what new features will Apple's apps have by then?
Apple's strategy of dogfooding for at least an entire year works well for them. But TouchID, NFC, and Siri are just small examples when compared to the Apple Watch, which give first-party apps huge advantages over their competition.
Maybe competing with core, first-party apps was never a great idea and Apple's apps may always have an unfair advantage.
All I know is that I'll definitely think twice about using a third-party app to replace a first-party one in the future. Now excuse me while I go set up Calendar.app.