Casey Winters (ex-Growth lead at Pinterest, GrubHub, Eventbrite) defines product-market fit operationally: customers reach PMF not when they stop being unhappy, but when "they feel satisfied enough to stop leaving and they start complaining about all the other things... you should have instead of just stopping to use the product." The shift from churn to complaint is the signal — it means retention has stabilized enough that the growth slope is positive and can be scalably acquired against. This is the baseline state a company needs before layering second-product work like Five Types of Product Expansion (Easiest to Hardest) on top; without a positive slope, spending on second products just papers over a leaky core product. Related: S-Curve Evaluation (Next Growth Wave vs. Enhancer), Business Model Fit (Beyond Product-Market Fit).