I can but:

    - in-app purchases come with more disadvantages than a one-time purchase (harder to implement, more UI and explaining needed, restoring purchases is a confusing thing)
    - I don't want ads in my products
    - there's not much functionality to limit, since the apps are very focused on a single thing
    - App Store reviewers don't approve apps that limit functionality too much

The App Store way of doing this is with an additional $0 in-app purchase named "7-day Trial", but I tried it and it creates an incredibly confusing experience for users.