To ensure a feature is built successfully, you can follow these industry-standard practices: How To Build Feature Flags Like A Senior Dev In 20 Minutes
While users don't directly "decide," their feedback and requests are the primary evidence used by PMs and POs to justify a feature's existence.
In Scrum, the Product Owner is the ultimate authority on which features enter the development cycle. They prioritize the backlog based on user needs, business value, and ROI.