It feels like a hostage situation. Had you started to charge money for that app, the most demanding and unreasonable cohort of users would have become apoplectic and invested time into trashing you and the app. It's almost like that in order to start charging in that situation, you need to retire the app under that name and rebrand as a different, fee-based product.
Especially because if I had monetized it, it likely would have been a subscription model because that would obviously be the only way to cover the continued operating costs, and users have such an aversion to any subscription.
