> just serve images fast with a little nonintrusive ad. No login required. No way the official Japanese apps can provide significant better service than that.

Why not? Provide same experience but for logged in users with extra benefits that they feel like it's worth paying for, behind-the-scenes content, WIP, whatever.

There is always a way to stand out and provide a better experience, the very least because all people in the world don't want the same thing, and you can always find somewhat of a niche somehow.