Apple analyst Gene Munster, from his 5 year Apple forecast:

Over the next 10 years, we anticipate that AirPods will be bigger than the Apple Watch as the product evolves from simple wireless headphones to a wearable, augmented audio device. While both AirPods and Apple Watch should continue to grow, we see AirPods contributing about the same amount of revenue as Apple Watch by FY22.

The key is the word augmented. Currently, AirPods are simply a great pair of wireless earpods. But over time, as Siri takes on a larger, more intelligent role in the ecosystem and as Apple moves into AR, the AirPods will have a more central role, acting as a conduit to other devices via more complex gestures and audio commands, and piping augmented audio back into your ears.

Our best guess is that Apple Glasses, an AR-focused wearable, will be released mid FY20. This is based on the significant resources Apple is putting into AR, including ARKit and the recent SensoMotoric Instruments acquisition. We believe Apple see’s the AR future as a combination of the iPhone and some form of a wearable.

Apple Glasses and AirPods are a natural fit.