Oh, common, don't twist the question, that's disingenuous. Not "a premium version". Why is it a subscription.
Why do think making people make recurrent payments for an otherwise static non-service app is reasonable?
$60 for a "lifetime" version is super steep, considering it's in the walled garden of the App Store, can't be backed up, can't be re-installed if it somehow disappears from the store, etc. For a desktop software it could potentially fly, but not for an iOS app.