I would be interested in purchasing a full featured ToDoist app than paying an annual fee for the full features.
Todoist is a cloud-based service with certain ongoing costs that we must cover every month for the upkeep of it allowing you to have cross-platform sync, 24/7 accessibility to your tasks, reminders etc.
We may of course consider introducing a one-time payment solution in the future, but for now we will remain primarily a service. You're welcome to add a suggestion/request to our Votebox, though: http://todoist.com/Vote_Android/
I understand the business model. Maybe a mixture of some full features and advertising can justify a price point for purchase.
BTW, I'm a MacBook Air and Android 4.1 user and I'm digging the cross sync ability.