I don’t know how many other journaling apps can import JSON, or your app’s JSON files in particular.
But JSON is a widely used format for moving data around, and it would be pretty straightforward to for an app developer to to convert a JSON journal into whatever they’re using for their app. Think of it more like a handy interchange format – you can turn a spreadsheet into JSON (and vice versus), or a CSV file, or a group of database tables, or an audio transcript, or a news article, or a blog post, or an XML file, or …
But I’m right with you on app lock-in. Exporting is the bare minimum to me, but even that can be a headache (obviously), if the two apps you’re moving between don’t export/import your data to/from the same formats. That’s why I’ve never gone in on a journaling-specific app.
More than a decade ago, I started using plain text (including markdown) to store my data as much as possible, because I got sick of having to export, import, convert data, try to find converters for obscure files… it turns out to be very doable (as long as you aren’t working with a lot of video), but you do give up some of the more polished apps like DayOne etc.