It would have disappeared by now for starters, I dont know if is stereotype but i get the feeling the american public suffers from short attention spans, plus i think it would have been a game to make a fast buck and then move on to the next thing.
One of GW's strenghts (and some would say weaknesses) in my eyes is that they take things slowly and release new stuff all the time because they understand the collecting side of the hobby, i think if it had been an american company( or anyone else for that matter) it would have been like "BAM here's 40k and everything that goes with it,thanks for the cash see you later suckers"
There's no guarantee of that, but it's possible, yeah. Not like America hasn't produced any good IPs, but this is a bit different.
If it was produced by an American company, I think it would've gotten slammed for "intolerance, slavery, lack of humanity, xenophobia (literally)," and God-knows-what-else. I don't think it would've been shut down or anything, but if it got really popular, it'd probably be attacked and boycotted by whiny children (that's what I think of them). Most people wouldn't care, as is the case now, but God help us if it started appealing to "children" (anywhere from 10-16 year olds). There would be quite the uproar.
I'm American, though, and I love the Warhammer world. How popular is it in England? I know that's where it started, but I don't think it's much different than in the States. Some people like it, but the majority know little or nothing about it.