As I mentioned, people tend to forget that manga struggled for decades to get a foothold, but what really put manga over was anime. When anime - SAILOR MOON, DRAGONBALL Z and YU-GI-OH, mainly, with NARUTO firming up the rear - got regular spots on American TV, in syndication and on basic cable channels like Cartoon Network, they created fans, the fans sought out the manga, and because stores began giving manga its own section, it became easy to for those fans to check out other manga, word spread, sales rose, and "suddenly" manga was cool, and being read by lots of people, especially teenagers, who had never even thought of opening an American comic. That's what propelled manga. Prior to that, Viz and other manga publishers had followed the traditional route - dump their product in the American comics stream and hope for the best - and made only a little inroad. Manga's content hadn't changed, but the manga experience had.
So any foreigners eyeing the American comics market, seeing manga's success, and figuring this means American comics readers are now hungry for foreign material are in for painful, especially financially painful, disappointment. (American publishers who make the same leap of faith will too.) All manga proves is that you can crack the American market, now that it's likely. And you'd better be in it for the long haul.