How do you separate a genuine product gap from a random feature request?

It's so easy to panic when a paying user asks for a specific workflow addition worrying they'll churn if you don't build it right away. But if you give in every time, your simple little tool slowly morphs into a heavy enterprise monster that nobody enjoys using. We started pushing back and asking what problem they were actually trying to solve instead of building what they asked for.

When a power user demands something complex, how do you politely say no without losing them?