Now, really, the way Jason Aaron had structured the story, there were only two suspects: longtime Thor supporting character Jane Foster, who is currently fighting cancer, or SHIELD agent Roz Solomon, whom Aaron created early on in his run back when Thor was of the Hemsworth variety. Most people I'd spoken to about it assumed it was gonna be Jane, so Roz ended up being the obvious but sensible twist, but then Aaron hinted at Roz too much, so Jane ended up being the new obvious but sensible twist. Either way, it was gonna be one of the two of them, and it did end up being Jane.
Any time you replace a superhero, there's bound to be a certain amount of backlash from longtime fans. For reasons that are probably sociological and too depressing for me to think about, that backlash seems to multiply if the replacement hero is a woman or a representative of a minority group. I don't quite understand it, as replacement heroes have been a trope of the genre since, at the latest, Barry Allen showed up in 1956 to become the second Flash. (Can someone name an earlier incident?) But it's easy to get jaded as a fan of superhero comics, since the only real status quo is change and eventually you probably just want things to settle down. Thor being replaced has also been a thing since Red Norvell in the mid-70s, so I can understand being dismissive of what one might see as a repeated plot. But all plots are repeated, right? It's just in the execution.
Jane has been the subject of quite a bit of "Why is she worthy all of a sudden?" conversations, but even ignoring the fact that the story has made it so that even Odin, who placed the enchantment on the hammer, cannot himself lift the hammer, the question for me is, why wouldn't she be worthy?
Think about it. In Marvel lore, Donald Blake is the human form of Thor Odinson, created by Odin so that Thor, not knowing he was Thor, would learn compassion and humility. It's in his guise of Don Blake that he once again becomes worthy of Mjolnir. In other words, Thor, the Asgardian god and warrior, stopped being worthy of Mjolnir once and earned the right to lift the magic hammer again once he started saving the lives of mortals as a physician.
The first time we're introduced to Jane Foster, she's a nurse working under Dr. Donald Blake. Over time, she became a doctor herself.
Her whole life has been dedicated to helping people. When you consider that Mjolnir has been lifted by Eric Masterson, an architect whose "worth" seemed to be based on helping Thor and almost dying a lot, and Dargo Ktor, a rebellious kid from the 26th century, it's really a wonder why people like Spider-Man or Carol Danvers can't lift it. But those guys, and Jane Foster, certainly have the credentials those others have — they want to help people. They're selfless.
|This guy can lift Mjolnir. Really, given these|
standards, Back Issue Ben would be worthy.
In the 10th issue of What If...?, from 1978, the premise is that Jane Foster finds the hammer of Thor. It's not "What if Jane Foster were worthy of the power of Thor?"; it's what would happen if she found it. She does, and she gets turned into the Goddess of Thunder.
She also has a romance with Odin, which, ew.
So really, the only question here is if she's actually ever tried lifting Mjolnir? And even then, it's irrelevant — even if she did, in 50 years of publication, contradictions are bound to arise. Seriously, there's a Spider-Man story by Stan Lee and Steve Ditko early on in Spidey's career that has aliens (it's the same story that introduces the Tinkerer). It's widely seen as a misstep in that legendary run. Roger Stern in the 80s wrote a story that explained that the aliens weren't really aliens, but no one remembers that story, so basically, the original story is still just a tale of Spidey fighting aliens that doesn't seem to fit well in Spidey lore. If a story did exist with Jane trying to lift Mjolnir and failing (and really, you're welcome to point me to it if one exists), all Jason Aaron has to do is ignore it. Or, you know, bring up the fact that the enchantment on Mjolnir changed so that even Odin and Thor Odinson can't lift the thing, and now Jane can.
There are stories where the hero gets replaced and aren't executed well (I'm looking at Dargo. I hate Dargo so much). There are stories where it works (Reign of the Supermen gave us three — THREE — usable and sustainable characters, Beta Ray Bill has lasted for so long, Bucky Barnes as Captain America is one of the greatest stretches in the history of Captain America). But it's not the fault of the concept (Dick Grayson has replaced Bruce Wayne as Batman twice. Once was well done and lasted a couple of years. Once was not that well done and barely anyone mentioned it again). And in this case, the structure is sound. In this story, Jane Foster is worthy of Mjolnir. And in my eyes, she probably always was.