When, other than in comics, does any character not have an obvious expiration date? In movies, you get maybe a trilogy - six hours of the character's adventures. In novels, the same - the short story genre you may get 20 pages of a character! TV series typically run for seven seasons or less, but often only one or two seasons. In no medium other than comics does time get so stretched such that the character can be the same, basically forever. So, the idea that people cannot commit to short-run characters is a bit weak.
This is not to say that the gender or race swap is preferable to having a full-fledged hero of another race or gender.
But, let us be clear about something else - it isn't like Marvel doesn't have female characters, or characters of other races. Asian, Hispanic, Indian, and Asian characters all exist, and have for decades. Female characters exist, and have for decades. Depiction and use of these heroes have been imperfect, I grant you - females have been depicted as eye-candy, members of various races have been stereotyped, and so on. But, as the times have changed, so have the depictions of the heroes. And, arguably, Marvel has generally been slightly ahead of the culture curve in their use - not *far* ahead, but slightly.
There is an issue, that these characters do tend to be "B-list". But Marvel's only partially in control of who is on the A-list. It is, for the most part, a popularity contest. Wolverine, for example was never supposed to be a big deal*. But, the readers loved him, so he got more and more spotlight, until he became a commercial driving force for the X-Men. Marvel is always in hot competition with others - they listen to their sales numbers, and they adapt. If they are sluggish to put such characters to the fore, it is at least in part because the audience doesn't respond to them.
So, how much risk is the company supposed to take on moral grounds? How much are they supposed to respond to critics when the audience isn't buying?
I'm perfectly cool with characters being introduced and failing or succeeding. What I'm not cool with is the whole second-hand nature of so many of the more recent attempts.
It's like our heroes come from the thrift shop.
If publishers want to make female/minority characters, I'm all for it. But why must their origins be inextricably tied to having a white male predecessor?
I think, if you take a real look at Marvel over history, you'll find far more female and minority characters starting out with their own identities than borrowed ones.
Marvel has tried, many times over, presenting characters who were not inextricably tied to a white male predecessor. But, historically, they didn't generally take off enough to be A-listers.
If you want to see minority characters, there must be action to support *sales* of those characters when they are presented.
It isn't enough to say, "It is bad that they don't have enough minority representation." That must be coupled with, "Hey, folks! Here's some minority representation - let them know you want to see more by BUYING!" Moral change comes most quickly when it is also good business.
In the meantime, Marvel does what it can - if the current minority buying patterns are not sufficient to carry a hero on their own, they'll loop in majority fans by linking to a title that already has good sales. The run of Sam Wilson as Captain America will sell. And lots of white boys will see a patriotic black hero. And they'll see a strong female hero in Thor. And those are good things.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.