This is exactly why legacy characters are created, only a small portion of it is the companies fault. Most of the fault lies with the current fanbase, as they only purchase what they've been purchasing since childhood, leaving new characters to most of the time travel to the land of Limbo unless a writer has an interest for the character and tries desperately to make a pitch (*cough* Rememder and Brother Voodoo *Cough*) That's why I never sided with that idea of just creating new characters. If new characters sold, they would be making new characters. But the current demographic does not support new characters. In fact, they barely support legacy characters unless some major press is placed behind them. Notice how every success story for new characters that came out is from a legacy character, and many of them had to replace the character which already has a demographic.
That's why I say the comic book companies in a way are very smart. They know very well the current demographic won't support new characters or minority characters if they have their own book. But comic book readers are some of the biggest creature of habits I've ever seen, they will continue to read a book even when they despise it. As such, by inserting that character into an already established demographic, this seems to be currently the only way of success, as folks who were picking up Thor before, are not going to stop reading the book just because a female may replace the hero temporary, no matter how much they hate it, because they have to own the series. It's Marvel playing the cards they are given.
With that said, neither company is without fault, DC much more than marvel in my eyes, and that's because IMO, neither company is trying hard enough to bring in new readers. With new readers who weren't originally comic book readers, you have a greater chance of readers giving new and minority characters a shot who are original characters. For my upcoming comics, I'm not even going to try and go after the comic book demographic due to how restrictive their taste are. Now Marvel does try quite a bit more to get new readers by going on the View and the Colbert Report discussing Thor and Captain Falcon, but they can still do more to try and gain new readership.
That's always been one of my major problems with DC, they tread much more lightly they seem with their characters than marvel does. But I also believe DC fans appear to be quite a bit more traditional and more resistant to change than Marvel fans. Hell, as mentioned before Val Zod could be the next big thing but a lot of internet fans not only claim for bringing back the original Earth 1, even though that would completely remove all the progress DC has made, many talk about reverting back Earth 2 to it's pre flashpoint settings. This is one of the biggest issues against the diversity as this resistance to change and something new is exactly why DC characters seem to be stuck and not able to move forward.It's possible that DC may be a little hesitant to give a minority character a higher priority than a white counterpart even if they see it as an advantage.
I notice other negative comments about new PoC heroes with original white identities can come negative at times. There's been other derogatory comments about Sam being Cap throughout the web (not in the CBR forums), which is sad. Despite all of this, a minority hero reached #2 in sales, which is damn impressive and no one can't deny that.