It's a shame that it already closed, I think some better publicity would have been great and given Spirits a stronger chance at actually placing decently while the thing was still open - the votes are definitely there, the timing is just all wrong.
Anyway, I think there's a market for this sort of thing in America. It's easy to be pessimistic and say people might not buy official products, but there's a sort of validation in owning a physical copy of something related to an interest a person happens to have. At the very least, the first volume would have sold decently I think. It's not like the DVD releases that tokusatsu has seen in America that are fairly expensive, a manga volume isn't often a wallet buster and spacing the releases out to once every three months or so seems to be the general practice and that works well enough for other series.
Though that 10% statistic doesn't sound too off from reality, my understanding is that companies always look at fan support online as only translating to, at the most, 10% of those fans actually buying the product. At the same time though, they wouldn't be the only audience out there. I've gotten into a lot of manga series by just having the cover art really captivate me and I think Spirits could do that.
Hopefully it does eventually find its way to the States, either from Vertical or someone else, but I definitely believe there's a market for an action series with a pretty strong story.