I could see it having a longer plot, tbh.
The end of the oneshot touches on the "real" message of the manga, which isn't just about a kid trying to get his older cousin to model for figures, it's about him having been inspired to follow his dreams by her when he was younger, and him wanting to pay her back in kind by making her remember hers ("If you're gonna say something like that, you gotta own it"). That's why there's the focus on how she doesn't have anything at her house, why she works a dead-end job at a hostess bar that his mom got her, why he brings up how brightly she used to shine as an idol even if they weren't super popular, etc. He's trying to get her to model for him, but more importantly, he's trying to make her feel like she's not just a washed up 30 year old whose life's already over with no dreams anymore.
Of course, none of that's explicitly stated, but it's easy enough to extrapolate from the ending + the overall tone. That, plus adding in the kid's side of things (social situation, his own dreams, his seemingly almost-romantic feelings he got for his cousin as a kid) would definitely be enough to make an actual series out of, even if it's not ultra long.