Well again, none of those other wielders called themselves Thor other than Masterson who was physically Thor so he is legitimate. Beta Ray bill, norvell, thunder strike and co. all held Mjolnir but never really used it. They each got their own mallets and hammers. And Earth/universe X is an alternate universe which doesn't count in canon. Just like the What ifs.
We dont even know yet if She will call herself Thor yet. All this is speculation and Thor is still a heavy part of the book. In my eyes, all this is is a story arc and a struggle for Thor. Another battle he must fight and overcome.
"Dear World: the nation of mutantkind is watching you. Do not #$%& with us." -Cable-