I'm a little surprised to find myself disagreeing with most of your points here.

First off, it is not really fair to just lay out the negative aspects of all the Slytherins like that, especially when ALL of her characters had rather ignoble and selfish traits. Let's consider some notable Gryffindors: Ron was shallow and downright peevish at times, Harry had cruel moments, and I have to agree with The Shrike about Percy, who was possibly the least sympathetic character in the books.

Then we have Dumbledore, who was sorted into Gryffindor yet exemplified so many Slytherin traits. He was extraordinarily manipulative, believed the ends justified the means, and seemed far more interested in secret power/control than in the trappings of Gryffindor glory. He was willing to sacrifice Harry in order to achieve what he considered to be the victory condition. Yes, everything worked out and Harry did survive in the end, but there were countless moments when that was not guaranteed, not to mention the layers of trauma.

Snape? Was not willing to sacrifice Harry; did not even want to let Dumbledore (an adult, an elderly soldier) die; yet sacrificed himself without a complaint. Seems pretty damn Gryffindor-y.

It's possible I'm giving Rowling too much credit here, but I always felt like the main point she was trying to make with the Hat was that we all have some of each House within us. Not all in equal quantity, but still present. And the House you are placed in perhaps has more to do with the way you try to represent yourself than it does the innermost contents of your heart. The two do not always match, especially when we're young. Still, the label you choose goes on to shape your experiences and reputation.

I think the popularity of Slytherin may have something to do with this idea and the fact that we all have at least some Ambition/Cunning/Selfishness, but Slytherins are the ones who deceive themselves about it the least. And this appreciation for authenticity (even when it's ugly and dark) seems to be gaining wider cultural traction. Maybe we believe that acknowledging our flawed humanness could actually be helpful to society? The alternative, where everyone tries to live up to an impossible ideal, does often result in closeted horrors.

I also think that Rowling did not so much fall victim to an overly simplistic Slytherin=EVEEL trope (except for what Paul said about The Battle of Hogwarts; that part definitely was oversimplified and frankly cheap). I actually think it may be that she felt a bit too compelled by her own darlings, namely Snape and Draco. Her disposition towards the Snape character arc puts her very much with the Snape fangirls, and I suspect she had a lot of sympathy for Draco as well. Whether her vision translated perfectly, whether you agree with the balance of virtue vs. sin - that's a different question. But I don't think she hated Slytherin at all; I think she probably loved both Draco and Snape enough to think they filled the need for complex Slytherins and even went a long way towards redeeming the worthless Salazar (and Voldemort, naturally).

For full disclosure, I am a proud Ravenpuff. I know I have Slytherin traits within me, but I don't tend to think they're a huge part of my overall makeup, nor do I really full any pull to identify with them more (of course it's also possible I'm lying to myself and/or am a self-righteous fool ). However, I agree with Paul about needing proud Slytherin-types in society. Just, you know, maybe not quite so many need to be lawyers.