I think the problem with Ariel/Eric is less with Eric and more with Ariel. From her perspective, the relationship feels more based on infatuation than any other animated Disney relationship. Yeah, characters falling in love at first sight is standard Disney affairs, but I don't believe Snow White ever considered giving up her voice and her relationship with her family members (assuming she had any) for a chance at that hot Prince body. So when Ariel becomes almost comically infatuated with someone she knows nothing about, it's reminiscent of a teenage girl wanting to stalk their favourite boyband member. And since she knows Sora pretty well by that point, the possibility of shipping her with him instead sticks out pretty noticeably.
I'm aware there's the argument that Ariel wasn't only interested in a lovely royalty shag, but also wanted to explore and be a part of the human world, especially since she has an entire song bout it. Arguably 2. But because romance is such an integral aspect of classic Disney, the non-romantic goal is somewhat overshadowed.
There's also the fact that if Ariel wants a gateway into the human world so badly, she already has that in Sora. If the story didn't have to take the same route of the movie, Sora could just go on land, bounce back into his human form, taking Ariel with him and they could go on cute dates together. I'm surprised I've yet to see fanfiction.net tackle the possibility actually.
However, I won't pretend I don't also see the appeal in giving Sora & Ariel a relationship akin to siblings, especially with how young Sora feels by comparison in KH1. Yeah, I know there's only a 2 year age difference, but it doesn't feel like 2 years with those characters, does it? And since Eric is by all accounts more conventionally attractive, especially since I'm fairly certain Ariel isn't a weeb, the decisions she goes for do ultimately make sense.
Thinking about it, there's a half-decent argument to be made for an Ariel/Riku romance; in addition to everything that applies to Sora, if Riku's muscles and height are any indication, he clearly fits more with Ariel's type.
But analysing Princess/Prince romances is stupid, just look at the pretty people in pretty outfits dance and fangirl.