Just one question. Why is aliens invading the old west any more preposterous than aliens invading the modern day? The answer is that it is not. Its just not a pairing that most people are used to. That's what makes it original.
I also like the serious tone. Why is it strange to have a serious tone in an alien invasion story. It's not. But people see aliens in the old west and it must be a spoof.

I have noticed that people are less likely to accept a movie that crosses genres unless it is a spoof. Then they are happy to go watch the silly movie. I haven't managed to figure out why though.
Cowboys fighting aliens? Not preposterous. Cowboys having
any chance whatsoever fighting aliens? A bit preposterous.
Not that in the grand scheme of things modern-day civilization would have that much greater a chance but at least we've got high-powered ammunition, jets, rockets and nuclear weapons at our disposal. Cowboys have six-shooters.
I'm not agreeing that the idea is preposterous, I think this movie looks like it'll be fun. Just that I can see why it's "more preposterous" than aliens invading present day. Really, the Aliens should be able to make short-work with 19th century humans.
That's valid point, although realistically, unless the aliens were barely able to reach the Earth to invade we shouldn't stand much of a chance either.
And they they gave him the glowy wrist gun to even the odds a bit.
Sorry if I came across a bit stuffy but the lack of willingness to accept cross genre sci fi is one of my pet peeves. When Event Horizon came out I thought it was a neat idea. The movie had its flaws, but I enjoyed it. Most of the commentary on it that I saw, however, concentrated on how stupid the idea that a space ship could go to hell. In a similar vein was the movie Doom. A truly bad movie, but the people making were so afraid of cross genre that they decided the marines had to fight ancient martians rather than deal with the scientists opening a portal to hell.
I think that there are some good ideas out there that never get a fair hearing because of the lack of willingness to consider combos of sci fi and other genres. It comes from both the audience and the filmmakers.