Good points. Yes, it's very much about the technology. Black and White is primarily a cost consideration. It's much cheaper printing in BW.
In the America there was a boom of indie Black and White comics during the 80s (eg. Teenage Mutant Ninja Turtles), but since then almost all American print comics are in full color. In Japan, they still publish the big manga magazines which usually have hundreds of pages, so I think they still use BW to keep them affordable. In America, single issues are actually comparatively expensive, using full color and nicer paper for fewer pages.
But, yeah, in a digital age, there's not cost difference to produce BW or color like in print.
I've been debating this issue for myself, because I was primarily print oriented: I was making a comic book that I wanted to get printed cheaply, so I was making black and white comic book formatted pages that I re-purposed for web comics. But I'm beginning to think that I should just forget about getting it printed and just stick to digital and make it in color.
I just love manga so much (and fantasize about being a mangaka), but I think I really need to get with the times and reality (ie. working in a digital medium and living in America)....