I’m sure there is or will be an export function. Can someone verify? I’m sure Apple knows it’s possible they may someday sunset the app, so I don’t think they would not give users that option. But I suppose they could not add the export function until they plan to discontinue the app, in which case then yeah it will glue those users to iPhones. But I’m inclined to think there will be an export function sooner rather than at the end, if not now then maybe when there is a Mac version.
I don’t know if it’s certain though that one will be required to update the phone every few years (3 years?) to use the journal app as opposed to updating at the end of the life of the phone due to other reasons (6+ years). Even if the phone doesn’t support the newest iOS, the journal app should continue working. There may be some feature fragmentation between your other Apple devices if those are updated though. But I don’t know if that’s too different from third party apps. Maybe some third party apps don’t even support older versions on older OSes.