A big reason we, and most other subscription based services, chose this business model is because of our online components. In short there’s an ongoing cost for us managing for example the key sharing and password manager. If we took a flat fee, it wouldn’t take long until we’d actually make a loss on every user.
However, I understand subscriptions are not for everyone, and you might be right in that there are people who would make a purchase if the model was different. All we can do is trying our best at making our subscription offer an as good deal as possible.