Quite the uphill battle for a developer.
This is different territory compared to marginally useful 2D app that millions of developers produce.
To make a believable virtual/3D experience requires enormous investments if you don't want it to look like a video game from 25 years ago. It's quite telling that Apple rushed through the 3rd party demo session at the event, as every example looked terrible. As terrible and useless as 99% of AR apps produced thus far.
I would suspect that only giants like Disney could produce a "killer app" experience that woes people and make this worth the money and discomfort.
And on top of that, yes, lots of developers will produce what are basically 2D apps where they take an existing app and project it in space, possibly showing multiple sub screens. Slightly more useful if done well, at best. Financially likely the only realistic scenario for most developers but it also cheapens the appeal of this device.
Not to mention that you need to apply way more real world scrutiny to possible scenarios. As an example, somebody suggested a cooking app.
To solve what problem, exactly? Cooking is instructions and timers. Yet with this device one can project more information. What information, exactly? Cooking requires free movement, to grab items, throw things in the trash, but here you are with a thing strapped to your head with low visibility. Possibly wired as you need to be thinking about battery during a long cooking session. Maybe you're cooking with two people, as couples sometimes do, then what? Countless real downsides, merely theoretical upsides.
Don't get me wrong? Will the tech elite embrace it and will a few influencers rave on about this cooking app on their channel? Sure. But that's not my point. My point is that you need convincing apps with tremendous added value to offset the price and discomfort of this device.
Without that, it will be solo movie watcher pro.