There is nothing automatically wrong with what you just said. However, you're leaving out a few important factors...
1. Increasing diversity doesn't require *taking* something from someone else. They didn't have to take those beloved characters away, unless they wanted to make people angry in the process. Which has nothing to do with story telling, and a lot to do with
getting some free advertising. So, that's strike 1 in terms of shaking trust in Marvel's integrity with these changes.
2. An updated reflection of society is good. But straight, white, males still belong in this society, right? So they shouldn't be treated like their presence, or unmitigated starring role, is somehow a negative. (Could you imagine if Clint went around with everyone calling him "the better" Hawkeye? Or if Spider-Woman gave a major press conference only to find that her fly was down a la Parker? Or is Sam Wilson was revealed as a Hydra agent this whole time?). Plus, if Marvel was just trying to keep up with demographics, they would actually have to do some rebalancing in the opposite direction if you talk about strictly who is *starring* in the books. (
Source) That's strike 2.
3. If you can't think of any counter-examples, then it probably is more than just the random creativity of writers trying to make good stories, don't you think? Unless Marvel has only employed writers who want to tell one kind of story. Either way, that's strike 3.
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So, it's not to say that the things you're voicing desire for are inherently wrong. They aren't. But when you step back and try to look at it all more scientifically, based on data and social psychology, it comes off very..... suspect. And so it stings all the more when a fan of someone like Lee can't stick around for 6 months in a starring role. It has nothing to do with just wanting to see a straight, white, male character.... Eddie Brock is all those things. But Marvel's willingness to be rigid about other characters, and completely fickle with one like Lee is very frustrating.
I hope you can understand that. And if not, hey, that's fine. We can agree to disagree - no problem.