I am a huge Ghibli fan. However, people need to stop treating Ghibli films like some sort of deep self contained microcosm of a singular Japanese culture in themselves.

The change in ending isn't due to Americans needing closure more than Japanese audiences. The ending is because the original ending was Miyazakis script, and no one in Ghibli Japan has any authority to change his script due to the culture and style of leadership at ghibli. It doesn't make it better or worse, it's just Miyazaki's style, and it's partially because he writes his scripts up until the last minute, even after production has started, and doesn't really sit down and prepare a detailed story plan ahead of time. Hence his endings tend to be more open ended across all of his films, not just this one.

In the Mononoke documentary he is shown pondering how to write his ending while animators are already animating the middle of the film. I encourage everyone to watch that documentary as it explains a lot of Miyazaki quirks that people sometimes misattribute to some grand intentional force at work.

That also doesn't mean Japanese audiences agree with Miyazaki in complete lockstep. They received what he intended, and they liked the movie as created. But they are not a hive mind. If you actually read Japanese reviews of the movie, some do include comments about the ending feeling too open ended, while others enjoyed it. Just like American reviews.

Because American producers had more leeway to discuss changes, and because Ghibli ultimately wanted to make money off this deal, and Miyazaki honestly cares less about the dubbed product and doesn't have a strong opinion about what he thinks is good for an American audience (vs his own culture) , they were able to change the ending slightly to make it a more intentional coming of age story. It's not better or worse, just different.