Never throughout the show did I pay attention to the soundtrack until the final episode. The tracks inserted when Hiro and Karada resumed their bond as siblings and when Tetsu confesses to Karada were the most outstanding and alluring, meaning that all of them made their chance to start over.
I cannot stop thinking through that the writers rushed the conclusion for Tetsu to finally accept that Satou was Karada all along, but it came well to transition to the parts where Shouko became honest to Kotomi that she had embraced her inner "child"; for Hiro to act like a man and become honest to Karada that she's always welcome to be her family sister; and for Karada to grow braver and wiser to overcome whatever changes must come in life.
My interpretation is that the two girls could not return back to their original ages, until they have reflected and fixed what either went or was going wrong with their lives.
Karada was afraid to move forward, because she wanted for things to remain the same. On the other hand, Shouko was afraid to return to the past. Why? Because both girls felt an aversion towards change itself, and the pain it causes or has caused to them when passed by.
Once they stopped going around circles and stepped towards the new tomorrow they were able to become honest with themselves and earned the wisdom to change themselves as well.
However, this episode was not without its flaws. I was a little bit disappointed that the conclusion ended just like a regular and generic TV episode that does not forms part of a larger story arc. The ending was put matter of factly, in my opinion. However, I was happy to see that the staff customized the ending video footage sequence to include Hiro and the adult Shouko having resumed their old love, and the picture of all three of them-Hiro, adult Shouko, and chibi Karada-now living as a real family.
