SEO without usability is soulless. The worst "SEO"ed sites may make a little affiliate money, but they don't make sense to users and they can't add to a sense of quality online that has helped e-everything become increasingly useful and trusted.
Design without usability is self-indulgent. The worst "beautiful" sites are invisible to search engines and unintelligible to users. They can be like a treasure hunt without a map, presented in a visual language that new users don't understand.
But, can usability do it's thing without SEO or design? Probably not.
Usability is the glue that keeps it real, and that's why I think the usability forum should be at the top.
But, can usability do it's thing without SEO or design?
Without SEO, yes, depending on what part of "usability" design is being done. Functional testing, which is included in the kind of usability work I do, isn't worried about SEO because things like bogus phone numbers and error messages aren't anything engines are concerned with. We care about error messages, because there's a whole set of usability guidelines for them (that most folks don't consider), but engines can't fill out forms.
It's tragic to see what I see sometimes. Like a travel site I tested, where the bookings form was literally unusable. It was impossible to book packages, both for functional reasons (the form didn't work properly), but also due to the overall usability of the app. It was complicated. The UI wasn't logical, nor did it always make sense to the task being performed. (Like it gave hotels for those wanting to book campsites.)
The site ranks well and is a prime site for travel in the area it covers. To not have a functional application during the summer travel season is a serious blunder. Why they didn't get it tested before now, I have no idea.
So yes. We care so much about the long term success of websites that we stick the usability forum in the #1 position here, rather than hiding it in the corner