It's not like people were picketing outside of Marvel, those concerns were made by fans who have seen any number of examples where well meaning people who weren't minorities tried writing minorities and turned in problematic stories. This isn't to say that only white people can write people, only black people can write black people, or only women can write women...I can think of examples were this has happened and the comics were terrible. But there's still something to be said about having an authentic voice on a title when it's supposed to be describing a certain experience. And Rivera wasn't just some random person that wandered in off the street, she has writing experience, she just wasn't a good fit for the title...it happens. What looks like a good idea on paper doesn't always work in practice, that doesn't mean you shouldn't try. And some people aren't seeing a white guy writing a white guy the same way they see a minority writing a minority because some fans think that the mere act of a minority solo title is extreme political statement.
Bringing it back to the Sentry, does this mean that only a person who's had mental issues should write the title? Of course not. But a good writer who has a mature understanding of psychology and knowledge of the different eras of comics would bring something to the title that some who can only do good actions scene won't.