The real Thor was transformed into a frog. Walt Simonson didn't introduce an entirely new character, toss them the hammer and say "Guys, this is the new Thor. You don't know know who she is, which by extension possibly leaves out why you should care about her, but she's definitely a woman. That's a good enough reason to like her." If Steve Rogers, Superman, Wonder Woman, Beta Ray Bill, or Storm picked up the hammer and started calling themselves Thor, then what happens to their OWN mark as a character and what THEY established? They're trying to fulfill a legacy that cannot, and will not ever, be topped.