No, what I am saying is that the problem is being made specifically to make them not white men, not because the story requires change. Steve's loss of vitality and Thor's failing are sudden moments with quick changing resolutions — and that doesn't make the fans happy, because it smacks of event marketing, particularly when Marvel promotes the change as better (not just different) and that the new hero is inevitably equal to or better than the original (which, if that were the case, they would be a different hero in their own right).
The difference is that when Thor became a frog, he was still Thor. Loki became a woman, but was still Loki. Replacing Thor Odinson with someone else who is automatically Thor with the same powers despite being a different character is not the same thing — and makes them an impostor and therefore illegitimate on the original character's legacy of heroism because it means that Steve Rogers, Tony Stark, Thor Odinson… they are not "heroes," they are people who are acting the role of hero and can be replaced when someone better comes along.
The desire to lean back on charges of "racism" and "sexism" in today's comics' social media circles is what fuels the publishers to work hard to prove they are progressive, but that's not what is happening with a lot of the criticisms — particularly with Thor. Thor fans want to see any of Asgard's existing women promoted and given the spotlight to stand as Thor's equal, not a replacement.
The problem with your position is you are not taking context into account — you are slamming past and present together to make an argument that is not supported by the evidence.
Superheroes did not make changes "all the time" in the pre-Quesada Marvel. When fundamental changes occurred — such as Steve Rogers' loss of faith causing him to quit and become Nomad instead, or Thor's transformation into a frog — the characters themselves remained the same at the core, a key part of the core Marvel, and were always able to communicate that this narrative change was most likely temporary and the character would be restored through an act of redemption once they had come to grips internally with the situation that caused the change.
More importantly, when pre-Quesada Marvel noted a diverse fan base, they responded by creating characters that reflected those individuals better. The rise of Blaxploitaiton in Hollywood and the associated awareness that American entertainment needed more African American heroes led to the creation of Luke Cage; the rising interest in martial arts culture, which was not predominantly white elsewhere, led to Shang-Chi and Colleen Wing. However, each character was created to be a unique identity that enhanced the Marvel universe.
The present Ms. Marvel is part of that second tradition. She represents the modern American Muslim community and some of its issues, but she is a hero in her own right who is building relationships with other heroes as she finds her place in the greater Marvel world.
The present Thor, however, is not part of either tradition. She represents a need to appeal to a particularly vocal segment of the online community that is critical of the mysoginist tendencies of comics — and she comes in at the expense of Thor. Similarly, making Sam Wilson the new Captain America delegitimizes the legacy of the super soldier program because it means anyone can be Captain America so long as it fits the image that government wants right now; when President Obama is replaced by Hillary Clinton, does this mean Carol Danvers will become the next Captain America?
You ask "Why is it important that these corporate characters not address a certain audience?" but the answer is simple: they already do. Superheroes are, by their very nature, oblivious to the differences in the human race. What matters is who the heroes are and what they stand for. No one, until recently, complained that Steve Rogers was the Nazi poster boy representing racist ideologies — what mattered was that Steve Rogers represented an innocence and simplified view of the world that we lost since the Second World War, and that he was a commander who dealt with people on a human level. When he was called out on it, it was by newer characters who noted the problem (like Night Thrasher did in The Avengers) or by the government (since Falcon was made an Avenger to fit quotas).
When it comes to Marvel Comics, the whole idea of the publisher being unable to respond to diverse audiences is simply inane. The X-Men dealt specifically with issues of race and gender. Dazzler dealt with gender. But not every hero has to deal with it in the way the online community is now demanding. The whole point of the shared universe is to present different approaches through different characters — not diverse characters, DIFFERENT ones who may be diverse but who definitely speak to issues that affect us all.
I turn the question around and ask you and everyone else who criticizes the defence of the core ideals why Thor can no longer be about the brash prince who must learn humility if he is to succeed in the role he is destined to play within the epic tapestry or history? Why can't Captain America be the idealized representation of a simpler time to remind us of when we used to care less about politics/gender/etc. and more about what's doing right? Why is Peter Parker's inability to have a really good day make him ineligible to connect to readers because he isn't of the right skin colour?
Since when does a changing statistic in demographic identification mean that a portion of the audience can no longer accept that the actions of a hero — any hero — speak to the greater good of us all?
And, most importantly: Why can't they seek out other heroes who have different views to help them, stand by them, expand the Marvel universe — why do they have to be replaced by someone who doesn't share the same idealism that defines the character identity and role in the MU?