The strategy Marvel is taking towards building more diversity into its books is mostly taking the form of having female or non-white people take the place of white male superheroes.
Marvel has made diversity a costume changing affair.
This is not the strategy they held in the 1960s with the introduction of Black Panther, Power Man, and Storm. In all of those cases they introduced new personalities with new powers, unique origins, and original costumes.
They built up new heroes from fresh. It wasn’t simply putting on the costume of a preexisting hero, like they do now. I don’t know if the current strategy is a more dignified or effective, but it can certainly bring a potentially obscure character to the forefront.
And it’s easier to take a well known character like Captain America, Spider-Man, and Ms. Marvel, or S.H.I.E.L.D. frontman Nick Fury, and simply have a minority hold the title and the post, and then write a backstory which sorts everything out.
Introducing a whole new character of any color would require the time and effort to build up the reputation. It would likely mean a new set of superpowers, a whole new costume concept, and a fresh storyline.
This isn’t to say I don’t love some of these new characters. Miles Morales as Spider-Man placed a new concept of Spider-Man over a pre-existing one. Most importantly, there was an authenticity in the writing. And writing is at the heart of any good story, anyway.
But the issue remains. Imagine if Miles Morales shared an identity with any lesser hero than Spider-Man. Imagine if he came in to take the place of Falcon, instead, or even Johnny Storm. Less people would care. By holding the mantel of Spider-Man he’s given that immediate shot of prestige.
And it’s easier than having to build a character from scratch.