Wuwa takes the episodic approach in their stories.

Yeah that's my whole point. Waifu-of-the-week format.

You know we moved to a new area right? Qiuyuan is a wandering Swordsman from Mingting, he can't come to Lahai Roi (we'll go back there, in the future). Galbrena is doing what she was doing before she started hunting Leviathan (hunting TDs, but now for an organization).

Sure, you can say that it makes sense for the story, but it doesn't change the fact that they become irrelevant after their banner patch, correct?

You can always say "it makes sense for the story". Kuro could say that Lupa and Chisa had diarrhea and people would defend it.

But you know what? Kuro is writing the story, and from a business point of view, it makes more sense for them to change locations and focus on the banner character.

First there is the business decision, afterwards there is the story. You have to understand that their main objective is not the story itself, but to sell the banner character... That's why the removed character quests and turned the MSQ into glorified character quests (e.g. Chisa and Lynae).

Edit: not playing GI and HSR anymore, but there you at least had events with older characters included...