A number of these things do seem like blatant marketing gimmicks. But then the landscape for SO MUCH media has changed over the last 10 years. It is so difficult to grab the attention of an audience, let alone maintain it. I think Marvel is proactively trying to engage the audience with these changes. They've done big things like this before, like when they launched the Ultimates Universe. And you know what? The Thor news was such a big deal that it made the papers and major news casts. I think the marketing execs would take that as a WIN in their book.
And comics fans are well aware that this isn't the first time we've seen Marvel do things like this with their iconic characters. Steve Rogers has handed off the Cap mantle before. I remember when he got fired as Captain America, and was replaced by the guy who would later become the US Agent. Then there was the assassination of Cap, too. Plus, the powers of Thor have been passed on to Beta Ray Bill, as well as the Thor Frog before. So why not a woman now? Those things alone show us that the characters aren't necessarily sacred even to Marvel. And at some point they'll probably come around with the re-birth or re-launch of the classic character as everyone knows them.