We recently moved to a paid app model with a 30-day free trial available (from a freemium app model).
Understand that this is not as appealing as a free-forever product. We found that we had hundreds of thousands of free users and not enough paid users. After developing the app for many years under this model, we had to make some changes so that we could continue to fund the ongoing app development and updates.
The notification prompt can also be declined in the alert that appears; though we could make this more obvious with a clear 'Skip' button.