Is there a benefit to staying past the end of Retribution?

No, there is no benefit to not boarding the evac. After killing the Heavy Assault that triggers the evac's arrival, points can no longer be obtained, but can be lost due to wasting time and being downed.

The only reason a hero may choose to not board the plane is simply to either:

• Boost their own damage and kills stats in the tab leaderboards
• Or just kill lots of things for fun

However you stop being able to earn points after the dropship arrives (after you kill the Heavy), and so you cannot earn more stars.