I wish someone smarter and more articulate than me could talk about the implication of this ending to the manga's supposed themes. But I'm going to try.
At the beginning there was a big, literally written correspondence between "youth" and being "trapped" in the timeloop. As if even without the timeloop, they were already living their youths aimlessly. We took a pretty long time to see the reason for her becoming an idol, and her answer suggests that this was just because she was searching for something. And him from the very get go was unwilling to "become an adult".
There are tons of anime/manga about coming-of-age in which characters struggle a lot to enter the japanese workforce (and honestly, who wouldn't). But maybe this one is more about the responsibilities x "dangerous freedom" aspect of adult life? What I mean is that he is very afraid of how corruptible "humans" are (projecting how he is, or at least he believes himself to be), and crossing into adulthood would mean he gains access to a plethora of bad shit that can fuck you up if you're just in an hedonistic pursuit.
So he tries to avoid it. Not to avoid working, not to avoid the responsibilities, but to avoid the corruptible pleasures.
Perhaps this is a twist in a way? It's usually how someone, who has been living for pleasure their whole lives, have to learn to grow and be disciplined, respectul and so on. In this case, he already seems plenty functional and capable for working. For him, the test is how to resist the temptation, not to give in. In a sense, maybe those pleasures are more tempting BECAUSE of how reserved he has lived his life until now.
So we either assume that the timeloop is just an uncaring, random occurrence, in which I say based - we don't need an explanation, we're just here for the vibes!
Or in this line of thought, this is a sort of divine/meta intervention, or internal mental struggle, or anything unexplainable which aims to make them better prepared to deal with "corruptible pleasures" in the adult life. Since they both were already ready for the responsibilities, their hell needed to be the other side of adult life.
Now, do they seem more mature or grown after all of this? idk. Their first action together is to skip school. This could just be a fun callback to how they spent all their days, or maybe how they will manage to lead more moderate lives from now on.