The end might have influenced the theme of redemption. If Joel had made some morally grey decisions in the past, the end of their story could be seen as a form of redemption or lack thereof. It shows how actions in a difficult world can't always be easily classified as right or wrong, and the end of their story is where the audience is left to judge whether Joel has found redemption through his relationship with Ellie or not.
The end of their story might have reinforced the theme of sacrifice. Joel's actions throughout the story, especially towards the end, were full of sacrifice. His decisions affected Ellie and the overall story arc, and this end showed how sacrifice can have far - reaching consequences in a world full of danger.
It could also have impacted the theme of survival. Their story was about surviving in a harsh world, and the end could be seen as a culmination of all their survival efforts. Maybe the end shows that survival isn't just about physical endurance but also about emotional and mental strength. Ellie and Joel had to deal with a lot emotionally, and the end reflects how they coped with all that in the context of survival.