What happens to a stakeholder's project influence over time?

The project is very challenging and has been very trying on a lot of people. There are some key team members considering leaving the project and going back to their old jobs at the company.
Which of the following would be a key motivator to keep them on the project?

Reserve Analysis involves:

At a daily standup on the second day of the sprint, the product owner asks one of the developers to add more functionality to a product backlog item that was committed to in the sprint planning.
The product owner explains that the change is based on a discussion they had with a user, is critical, and should be delivered in the next release.
What should the project manager do?

A subcontracting company is presenting its feature release A key stakeholder is asking to add a new feature to the product How should the project manager deal with the new requirement?