I actually just replaced my ST with a 17 STI a couple months ago.
I did consider an RS, but dealers around weren't playing ball, and really it would've negated the main reason I was trading in. I needed more passenger space, especially in the rear (have a 2yr old, likely another on the way). I still wanted a small-ish car that was fun to drive, but had all the practicality I needed. The back seat space is so much more useful than the Focus st/rs.
But anyways, as far as image is concerned, the WRX/STI has been around in the US longer, so its gotten a lot more time to attract certain not-so-great people. As others have said, it has already started happening with the ST, especially with its lower price point. It's just a matter of time with almost any performance car and once prices go even lower in the used market.
The RS is indeed a better performer in almost all aspects for sure, and I'd have likely gotten one instead if the need for space wasn't a major factor. I loved my ST and the RS only adds to that.
People buy STI's because they can't afford a GTR. Same crowd in my book, just one of them makes more for a living.
LOL at the STI being a cut-rate GTR - I don't believe at all the the STI and the GTR are the same crowd. That's actually more the EVO with its more spartan take on these economy rockets. The GTR actually makes stupid speed accessibly easy. That's entirely the opposite of the STI, where you really have to work on it to get the most performance and speed out of it. The RS is actually closer to the GTR due to the newer tech, being a lot more forgiving, and much easier to go fast with (let's not even mention the Golf R with its DSG).
The WRX/STI is actually the more practical option, but its reputation (mainly due to previous owners) has preceded it in many bad ways. In reality, its a fun, quick, and quirky car that has awesome all weather capability with the right equipment. It's really not that far off from the ST/RS in terms of its mission - to give you a fun daily driver that can take nearly anything life throws at it. However, people took what it was and just kept pushing it to the status it has now.
Sure, the RS and the STI take a more track-oriented approach, but neither are true sports cars in my mind with the 'weight' of having to be practical as well. They're just that level-up from the ST and WRX. They are indeed substantially more capable and 'racey' feeling, but both still have economy/practical roots which hinder them.
That said, I like all of them and I totally understand the whole 'grass is greener' thing, but rest assured that your ST is awesome and holds its own in the wrx/sti crowd. I wouldn't say you're 'not missing out on much' - but its just a different take on the same formula. They're all fun cars with different attitudes.