I have been in a position where I have said no. I was able to stop the feature factory for a few months. But after that we were right back to cranking on features because several end users "validated our roadmap".
Santa Claus isn't going to fly down your chimney and deliver a fully formed business case. Developers share responsibility with other roles for figuring it out. Sometimes it takes multiple iterations of exploration and experimentation. Expecting to be told what to do is just lazy and unprofessional.
Because it's not about bridges or tunnels, common sense is thrown out of the window, people start building something they conjure out of thin air.
Few people say 'No!', we don't do anything. Anything. Until we have written a clear business case / usecase, a raison d'être for this project.
Agile is often abused to not think, not design and just follow the whims of the customer. Nice for billable hours, but long term?
Do you produce anything of value?