This is one of the reasons why kanban is my favorite project management methodology.

"Ah, new highest priority feature. Lets do a mini discovery session and it looks like this work can be represented with 20 sticky notes. Our historical team velocity is 10 sticky notes every two weeks. So we'll probably be done in four weeks. If we take down all the existing work in progress. Lets go to each existing sticky and you can confirm that we can stop working on it."

The stakeholders are still allowed to make decisions, but (hopefully) it forces them to be realistic and understand what they're asking the team to do.