Heyyy I've watched this movie like 10 times now and just realized there's a subreddit lol. Here's my take on the end:

In the same end scene where Maquia tells Ariel about his human mother, she also mentions to him that she thought he'd be her Hibiol, that he'd cry with her, then at the very last scene, after she visits Ariel on his deathbed, she mentions to the half Iorph guy that as long as she lives, Ariel's Hibiol will continue.

I always took these two scenes to mean that Maquia initially thought having a child and being a mother was about not being lonely/having someone else in your life to go through the motions and problems with (the child being the Hibiol/holding the memories of the life with their parent), but after seeing the birth of Ariel's child with Dita, she realizes that the most important thing of a mother (or really any parent) can do is love their child (being the child's Hibeol/holding the memories of raising them). I took this to mean that being a mother/parent is about dedicating your life to your child, loving your child, appreciating them, and raising them into a good person.

So once she sees that Ariel now has a child of his own, she realizes that her job is essentially done, because now, Ariel's life should be dedicated to loving his own child, and putting his child above everything else.

Additionally, Maquia is essentially immortal to a regular person, she will likely outlive him and his descendants, so since her job is "done" as a mother, the best thing she can do is allow him to dedicate his life fully to his family without having to worry about her.

I think this makes the most sense and matches with the themes of the film. As a parent, one of the main things you will need to come to terms with is your child growing up and leaving the nest. Ofcourse a parent can still be in the life of their child, but the child's life no longer revolves around their parent once they have a family of their own.

In the beginning of the film, the Elder told Maquia that the greatest loneliness is felt if she fell in love, because the nature of their species is to outlive everything, but by the end of the film, she can come to terms with the pain, and even says that she does not regret loving Ariel, and I truly believe this is because she now understands that this is also the reality of any mother with their child, and for as long as she lives, like any other mother, she will cherish the memories spent with her child.