Why fix what ain't broke?
First I don't think that it fundamentally changes who Thor is... and the Hulk underwent a major revamping into his origin (a failed Bioforce Enhancement project v. exposure to a nuclear blast). The failed experiment was more palpatable than the nuclear blast. I feel the same way about an embellishment to establish Thor a from another planet, rather then from a mytical otherworld... For comics, especially 40-50 years ago, that was fine... It is fine in the books today because of that history... But the movie could use a modernization of the backstory.