“If you do implement a hard paywall and Apple finds out about this, there's the risk that they'll move your app from the free category to the paid, which will really mess with your ASO. Duolingo... would never have become the company they became [with a hard paywall].”

Hard paywalls cap category-leader TAM and risk Apple reclassification

Hard paywalls juice ROAS but throttle the funnel a category leader needs, and Apple may shunt the app into the paid charts and crater ASO. Duolingo overtook the older Babbel partly because Babbel gated everything; Duolingo opened the top of funnel, then layered consumables, streak freezes, and ads on top.