While several people have suggested that someone in this position should be more assertive if they have an immediate need to use the restroom, as someone on the receiving end of such assertiveness I would always wonder if the person really had such a need or whether it was just someone being impatient. Sadly I know all too many people who would offer a fake excuse just to get what they want. I for one would generally give such a request the benefit of the doubt but I don’t know if everyone would. If the request came from the cabin crew that would help show that the request is legitimate while also helping people for whom such assertiveness is uncomfortable.