I actually never once had this impression–the opposite. I think Historia is excellently written as a character, as is Ymir–until her death which I still think was poorly written.

Both of them have arcs about learning to do what they want to do, instead of being codependent–aka unhealthily obsessed with proving their worth through serving others at the expense of themselves, because they don’t believe they will ever be loved. They are foils. Historia I believe will overcome this flaw (and made tremendous progress in the uprising arc), whereas Ymir tragically did not (a character does not have to overcome their flaws to be well done). 

However, it’s also true that the most important character in Ymir’s arc is Historia, and the most important character in Historia’s arc is Ymir and has always and will probably always be Ymir. There is nothing wrong with that. Mikasa’s primary relationship is with Eren, and Gabi and Falco’s with each other as well. That doesn’t mean no other characters or nothing else in their arcs matter, but it does mean that their relationship is vital to their development. It’s not forced or just in there as fanservice (there’s actually not fanservice/ sexualization of Ymir and Historia); it’s natural and shows how Ymir loves Historia because she sees herself in her, and believes she’s a person who deserves to be loved. And Historia sees this in her relationship with Ymir, because she knows Ymir did love her. It’s thematic, tying into many of the story’s themes about self-worth and abuse done to children and second chances and hope, even in the middle of the worst circumstances imaginable.

Ymir’s death was sad, but she still got that second chance at life and through her, even though she’s dead, I do believe Historia will get her second chance (and therefore be a great mom to her baby) as well.

If people talk about the ship a lot, it’s because there is a lot packed into that ship. Despite its ending, it’s how I believe an excellent romance can be incorporated into the main plot and only add to the story’s quality as a whole. And because of its plot importance and thematic importance and character growth imporance, it is really excellent lesbian representation which like, is sorely lacking in general media.