I completely agree with your point here about what the functional purpose of a user interface is. I would also agree that almost all
styles of design have the potential to accomplish all the functional goals of an interface,
except, perhaps, flat design.
Once you've adequately addressed the functional purposes of an interface, you are free as a designer to design it however the heck you want to. Usually the rest of these design decisions should be made through the lens of brand image and how you want your users to feel. The rest of the design process is practically a science (ie how the average person scans a page, perceives content hierarchy, expects certain UI elements to perform certain actions, etc). So the bulk of the truly creative portion of design is subjective, and designers should remember that. Yes, designers are always forced to be creative when solving UX and architecture design challenges, but ultimately the result is not subjective. Your objective goal is to help the most amount of users understand your website and accomplish their goals. When designing the style of your website, you're deciding how to influence people's emotions. An almost flat design like Google's is great for Google (and I truly mean that, I really love their design aesthetic as of late in their apps), but that's not because an almost flat design aesthetic would be great for every website. The almost flat design aesthetic makes me think of digital products, modern companies, and a subtle, friendly personality. This works for Google as it does for many other companies, but it might not work for yours.
Find your brand image. Craft your own emotional experience for users that is unique to you and your company. And when you find something you like, don't tell other designers it's better than their styles ;)