Do you expect those users to be sticky? The barrier to entry for making a to-do list is negligible, and having a long history of completed tasks is of very little value.
Fred Wilson wrote about user metrics, he called it the 30/10/10 rule (http://www.avc.com/a_vc/2011/07/301010.html) - 30% monthly active users, 10% daily active users and 10% new users each month. We can apply this rule to Wunderlist, but as we're in the 'task' market, we generally have more monthly users than daily users - because the product is more optimized for weekly use.
With 30ish people, let's say you burn $2 million a year. You'd need to get 30,000 people to each pay ~$70 a year for your app. My numbers might be wrong, but I'm just curious how you'd get people to spend that when it's so easy to switch to another task app.