For the majority of fiction, if it isn't interesting, it should be cut. Because it served no narrative purpose. Now Sylphie has been in the story since very early on, but there's also currently nowhere for her to go in the narrative. She gives birth, concluding her arc...which isn't actually HER arc, it is actually a small
portion of Rudeus' arc. There's nowhere further for her character to go.
Does Sylphie
need to stay at home for the narrative? Not really, that's why Aisha's arc has her wanting to be the perfect housekeeper and as the bastard child of her father with a complex about it, wanting to be just a
little selfish. But Sylphie's story got rushed right to pregnancy, limiting her utility in the narrative.
Does she provide a "true love" type relationship? Not really either. That's been set up to be Roxy. Heroic savior at her most dire need and everything.
Does she provide a "dramatic love" type relationship? No, that's Eris' role since we know she still loves him.
Sylphie couldn't be cut because she was a big part of the early events getting Rudeus to be more social. Again, Sylphie serves as a vehicle for Rudeus' arc, not the center of her own. Which is consistent about her everywhere she appears in the story. She's not a lead heroine, she's a tertiary support character to other, more interesting characters in the story.
Her entire role in the webnovel should have been rewritten
from scratch to make her an actual character, since they certainly couldn't have cut her. Even
Sara has more of an independent sense of development than Sylphie does.
The totality of her personality is horny, available, submissive, loyal, devoted, and obedient.
That's toxic social construct writ large. Flaws make characters interesting and engaging. Motivations that don't perfectly align with every other character drive the narratives. Boring is not a flaw, it is a lack of one. Sylphie never affects the narrative, she's carried along by it.
That's literally why she's more interesting, same with Roxy. They're active participants of the narrative, and Sylphie is barely even a supporting character in Rudeus'.
And you clearly
misunderstood the best MILF in the series. It's not well explained in the anime, but you can piece it together from her husband's dialogue concerning the Greyrat power dynamics (all moot after the mass teleportation) if you pay close attention. Eris became spoiled because she was the only child Hilda had left, but in that scene, Hilda also berates Eris for not being feminine enough (Hilda knew that Eris needed course-correcting, she just didn't live to see it). Hilda initially resents Rudeus because Paul managed to get out of the traditional Greyrat family dynamics.