So I'm thinking about how much of the basic vibe of Firefly is patterned after the American Frontier if the late 1800s. Whedon has explained why this is and I'm fine with that. But consider an alternative... We never saw Earth That Was, nor the civilization that directly left there. All we know is the aftermath. So why would said civilization set so far in the future replicate clothing, tech, customs, and speech from centuries past? Maybe because they didn't replicate anything. Say the 'verse is actually an alternate history where a Steampunk-inspired 19th century came into being and never fell, where East and West established a balance in the 1800s that continued into Firefly's present day. Some aspects of culture and tech advanced while others (six-guns, trains, horse-drawn wagons and cattle-raids) didn't. I'm not suggesting that Serenity was steampowered, although Kaylee's enginee room looked like it. I'm suggesting the opposite... what if the Old West (or an alternate version thereof) was nuclear powered? What other Jules Verne-esque variations might Earth That Was have held? EDIT... After closer thought it sounds rather silly. Please disregard.
Nuclear power is steam power, the radiation heats the water and the steam generates the power. Steam punk nuclear would have the steam from the nuclear reactor being pumped throughout the ship and directly running pistons that operated mechanical devices like elevators or large blast doors. Factories used to operate that way. It could be considered more efficient, you loose energy converting steam pressure into electricity, and then converting again into physical force. The efficiency makes more difference if you are operating heavy machinery like mining equipement or heavy presses, rollers, etc. Engines could just be steam further superheated into plasma exhaust. Another cool concept might be some kind of warp field FTL drive where steam powered pistons keep some kind of device spinning generating a powerful field, similar to an electro magnet. This is where the concept gets interesting, if our warp field does something to screw up electrical equipement, which is why our ship uses fully steam driven mechanical parts and manual switches. What I'm describing has nothing really to do with Firefly, but it's a fun concept to play with.
While Firefly had some aspects that harmonize with steampunk, I don't think that it's necessarily based on a similar conceit ... Don't think of it as 'replicating' these things. The Old West was as it was for perfectly good reasons, reasons that apply equally well to the 'verse. We saw that in the core, technology and culture were very similar to modern visions of what the future might look like, especially the current trend of making the future share many aspects of our own present, rather than becoming a chrome and spandex world of rocket packs and pointy helmets and shoes (take nuBSG, for instance). However, on the outer planets, they didn't have the resources, either natural or technological, that the core worlds enjoyed, so they had to make do with what they had or what they could make. If energy packs are scarce or prohibitively expensive, you find new ways to power your work, down to the point of building windmills. If you land on the planet with nothing more than the shirt on your back, a couple blankets and a couple head of cattle, and all you have around you are dirt and rocks and trees, you make a home out of those trees, you plow the dirt and plant in it. Without powerpacks for high-tech weapons - maybe you had to use the last of them to clear out a homestead and cut down some trees - you go back to tried-and-true gunpowder (or maybe some other mineral that is prevalent on your new planet, and which has similar attributes). After you've been raising crops and animals, you end up making homespun clothes to replace the ones you've worn out, that you no longer have a manufacturing base to replace. The technology, architecture and culture of the Old West is about the minimum to which modern men would have to retreat if they were forced to give up the levels from which they had come. And they might be able to preserve some of the technology they brought with them, so one would find bits and pieces of it here and there that would be inconsistent with their overall current lifestyle, but consistent with the way in which they started out, and were forced to abandon those things for which they no longer had a high-tech infrastructure that could maintain them. As for the language, it was always my view that what we heard was not necessarily supposed to be how they actually spoke, but was in fact a translation for a modern audience, in the same way we see biblical stories in which everyone speaks English (often with a British accent!). Cut off from the 'mother tongue,' both physically and philosophically, they develop dialectic equivalents to the speech patterns of the Old West, whether they actually replicate them or not, so in showing them in a teleplay, using a recognizable Old West accent and slang reinforces how they 'fit' into their society. Keep in mind this is 500 years in the future; language can change a lot in that time, even on a single planet - imagine how much it can change when its spread across hundreds of worlds. If they spoke as they might if all of this were real, we might not understand them at all. I know a lot of people complain about the conceit of the Old West setting in Firefly, but when you think about the circumstances that Joss and his co-writers set up for how these people got out there and what happened once they did, IMHO it's not only a logical result, but it would be unrealistic - and bad science - to be otherwise.
I do understand Whedon's rationale as stated in my original post, no need to explain it again. I just found said rationale to be rather boring and predictable. Whedon himself has admitted that he knows nothing about science fiction, he was more interested in the characters. All I was trying to do was suggest a more fanciful alternative to what was already given.
I think it had lots of elements of steampunk and don't think your post is silly at all. On Whedon being more interested in characters than sci-fi, as a sci-fi fan I have two words: THANK YOU! Science fiction is less about shooting stuff with lasers in the future and more about the circumstances of the world and the characters. We know through Serenity that Firefly is about a bunch of humans who went berzerk and how an "empire" overtook the galaxy removing the rights and liberty of men. That's pretty futuristic! In fact, it's the basis of Star Wars. And the characters were awesome, particularly Jayne who cracks me up. I think caring about the characters is the most important thing a writer can do.