Should I build the feature one loud customer wants?

One customer — a big one, the kind that pays for a whole engineer — keeps asking for the same feature.

Building it would make them happy, probably renew them, and might unlock a whole segment of similar companies I can't currently serve.

It would also pull my roadmap sideways for a use case nobody else has asked for, and I'd be maintaining it forever for one account, because the moment I ship it, they'll depend on it.

I've watched companies become agencies for their biggest client and quietly die. I've also watched companies say no to their biggest client and lose them.

Where is the actual line between "listening to customers" and "being held hostage by your biggest one"?

And if you did build it, did it open the segment — or just keep the one account happy until they left anyway?