While idea viability may be validated by traction (read: user acquisition), product viability is best validated by DAU / MAU and low churn metrics. That being

While idea viability may be validated by traction (read: user acquisition), product viability is best validated by DAU / MAU and low churn metrics. That being said, we closed $750K in my last startup after crossing 125K users, so perhaps traction is good enough at least for the first round.