Unfortunately, program managers often act like defense attorneys for their projects. They sometimes act as if their job is to keep the project alive, and defend it to all comers... even if the project is a dog and needs to be cancelled.
The program manager (arguably) knows the most about the health and viability of the program. If it's heading over a cliff, aiming for irrelevance or otherwise doomed, the PM is usually among the first to know, and should speak up. Sadly, this does not always happen.
PM's are not defense attorneys. Their job is not to keep the program alive and out of jail. It's to guide and shepherd the development. And if the program needs to be cancelled, the PM should lead the charge.