Presumably if people don’t care, they don’t fill in the survey.

That's not what I said. People care about the survey and they do a favor to Mozilla with it. And if a question does not have the answer they want to give, then it becomes a problem. It's a different scenario than what you were saying.

But yeah, this design has worked well for many other surveys, so presumably it’ll work well for this one. They’re the experts :)

With that attitude and without acknowledging a problem, it won't get better. If they were the experts, then they wouldn't need a survey. But its easy to discredit any credit with that dumb argument.