There's clearly an audience on the PC, unless the only reason that Touhou sells is the characters.
I would be very suprised if Japanese shmups couldn't find a home on the PC, especially with the many pricing and bundling options that are available. These games rarely sell many copies at retail, so there's a possibility they could do even better in a digital enviroment like Steam.
Do we know how many copies something like Jamestown have sold?