The video of me reading the following.
Diversity at Marvel Comics is a problem. Not because diverse characters are a problem but because the defense of those characters is acting as a smoke screen that masks the actual issues.
When someone complains about Jane Foster being Thor, the easy conclusion is that it is because she is a woman. But I remember when Eric Masterson took over for Thor, a character which was demographically identical to the original Thunder God, and people complained about that too.
When John Walker became Captain America, people complained. When Bucky Barnes took over, people complained. When Sam Wilson took over, people complained, but this time, because he's black the fans must be racist.
For that matter, Spiderman was replaced by a clone of himself and people hated it, even if they liked Ben Reilley as the Scarlet Spider. They hated Azrael taking over as Batman and were just waiting for the real Superman to return after his death. Heck, in that case Steel was one of the replacements that people sort of liked.
The best example of a mantle passing I can think of, the passing of the title of the Flash from Barry Allen to Wally West, still had people upset about Wally taking over as the Flash and decades later when Barry returned and Wally was ousted they complained yet again.
The thing you have to understand is that Peter Parker is Spiderman, Matt Murdok is Daredevil, Steve Rogers is Captain America, Thor is Thor, Clark Kent is Superman and Bruce Wayne is Batman. These are intrinsic links in the fan base's minds and to try and change those links means forcing the fans to complicate their language when discussing the characters. Comics are already a pretty complex thing to talk about, adding extra levels to it eventually kills the conversations. So while of course you can shake things up and switch things around, barring a lightning in a bottle event, a character history which supports it, or meticulous setup which will still probably be rejected, in the end the characters will end up more or less back to where they started, having lost some portion of the readership along the way.
Fans of comic books invest years and significant brain space to these characters. Over time a para-social relationship forms and a lot of the fans come to think of these characters as friends. That friendship is the thing that pulls them back week after week, month after month, and keeps them spending their money to find out what their friends are up to now. They are in a position where they have to trust temporary caretakers to treat their friends with the proper respect.
Would you be upset if you found out the people working in a hospital or nursing home were abusing their power over people that you care about? What if they wanted to send a stranger home with you while insisting they were your loved one or even worse that you were better off for the exchange? That is much the place that fans find themselves, when reading comics. While they will accept bad things happening to the characters and minor changes in personality and circumstance from creative team to creative team, if you change them fundamentally, especially in a way which feels disrespectful, then people get upset.
If you change a bunch of characters in a short period of time then the irritation is going to compound. I remember when the Heroes Reborn story happened and there was a massive hue and cry because it changed the characters on a fundamental level even if it didn't change who was who.
Another thing that the reactive defense of diversity is potentially obscuring is that most characters can not carry their own ongoing series and even those that can, may not be able to if they don't have the correct alchemy of writing, art and zeitgeist. If the fans can't say "I don't like Kamala Khan," without it being written off as racism, sexism or Islamophobia then how will you ever determine that the character, like so many dozens before her, just isn't interesting enough to support her own book? (I'm not necessarily saying that Kamala Khan is not a strong enough character to carry a book, just that jumping to conclusions about the motives of people that complain about her, is detrimental to an accurate assessment.)
With that said, a good enough story can ameliorate much of this negative reaction but if you have creators acting defensively, firing back at the (admittedly often mean and thoughtless) fans and accusing them of being bad people for not liking when someone is screwing with their friends then that is going to galvanize the negative feeling rather than defuse it.
There is no way to control how the fans are going to react when you create something and put it out for them to consume. There are too many of them to expect that they will all react in a reasoned and professional manner but since they are fans and not professionals this shouldn't be expected regardless. In the grand scheme of things you can only control yourself and as a professional there is an expectation that you will behave in a manner which reflects well on your employer.
That isn't to say that you have to agree with the fans, that you should give them what they say they want or that you aren't allowed to have an opinion. All it means is that when the fans go low and attack you or your work you behave in a civil manner. That might mean engaging them in a polite argument, having a friendly conversation or simply ignoring them and moving on with your life. If you don't make enemies of them, most of the fans will do the same but it is up to you, as the professional, to behave in a professional manner.
In addition to civil behavior reflecting well on you and your employer it will also serve to clear out the actual trolls who will largely lose interest if you don't respond to them in the way they want. Real fans (defined here as people who read comics who are not posting with the express intent of getting a rise out of you) may say things you don't like or in ways which you don't approve but their reactions to the work are still valid and can still be used to help improve future efforts.
So yes, I believe Marvel does have a diversity problem but it is because the knee jerk reaction to criticism of new and diverse characters and creators prevents an accurate assessment of the complaints, rather than a problem with the diversity itself.
