Apple has been emphasizing "services" in 2019 — it held an event in March devoted entirely to new services for news, gaming, and TV — and that's what iOS 13 is all about: keeping you locked into the Apple ecosystem.
The company improved many of its popular first-party apps like Maps, Reminders, Notes, and Mail, which are truly significant updates. But in iOS 13, Apple is really focusing on ways that the iPhone can improve other devices.
The new HomeKit software gives you more reason to buy a home-security camera, since footage can be stored in iCloud for free. The new CarPlay software will greatly improve your car experience. The iPhone now hands off to HomePod, and lets two pairs of AirPods work together so people can play games or watch movies on the same iPhone. The new Reminders app pays attention to your activities to proactively help you stay on top of your to-dos. The new Maps app, and the ability to easily share collections of places with friends, looks like a great reason to try the software again. Everything is designed to keep you in iPhone World, or bring you back if you've left.
So, the iPhone hardware may change in the coming years — it might have three rear cameras this year, for instance — but Apple's software will continue to connect the iPhone to every other iDevice, and even other products outside of Apple's ecosystem. The more things the iPhone can connect to, the better chance it has to compete against phones that are getting better and better every year.
You can read the full list of iOS 13 features on Apple's website.