Platforms only have killer use cases in retrospect.

You can't motivate turning something into a platform with a singular killer use case.

If there were one killer use case you knew would be a massive success and would work, you'd just do that.

You can have notions of classes of apps that you know will likely be great if you build the platform.

The point of a platform is the kinds of emergent things that show up even though you didn't plan them.

Seeking a singular use case to focus on gets you in "building an app" mode, not "building a platform."