As Albus emerges from the lake—thus indicating that their plan succeeded—it shows how Hermione, Ron, and Snape’s sacrifices were not in vain, and they were able to save the wizarding world. Scorpius’s delight at the fact that they failed emphasizes how wrong they were to try to change the timeline in the first place. The play thus suggests that even when presented with the opportunity to change the past, it doesn’t mean one should do so.