Well, it depends on what you consider decent. I don't know about other markets, but in the direct, monthly market, sales showed that even back on KSD's run Carol's book would approach the cancellation zone very early on. The difference is that the drops between issues were a lot smaller, making the sales a lot steadier... it would even get a sales bump every now and then, showing that the audience, albeit small, was loyal. So yeah, KSD was very good at keeping Carol's audience together... something that has been happening less and less, specially after the sales slump, which haven't done Carol any favors.
And while I'm the first one to point out how DM sales are not the full picture, I can't help but feel that the constant relaunches are evidence that Carol is not in a Squirrel Girl/Ms. Marvel kind of situation.
I don't think anyone expects Thor Odinson to be forgotten. That's definitely not happening. Which doesn't mean Jane will get the Thunderstrike/BRB treatment, because those are not the same situations at all. Those characters never got the kind of push or the kind of sucess Jane got. I'd say Jane was a breakout legacy star just as much as Kamala Khan. It's the kind of success that rarely ever happens with a new character (or in this case, an old character taking on a new role), so why shouldn't they milk that? People sharing mantles has become a thing at Marvel, so there IS precedent for Jane to still be Thor even after Odinson returns to the title. Also, your Strange/Voodoo comparison doesn't really work in this case, because Jane was definitely not shunned like Voodoo was. There are tons of writers and books outside of Aaron's work that specifically wanted to use HER as their Thor figure, even though Odinson WAS still around and COULD still be the Thor reference. People LOVE Jane. She outsold her predecessor AND is still smashing on sales, her book is critically acclaimed, she has a solid fanbase, she guest starred on multiple books, she became an Avenger (on the main title, nonetheless), she's been adapted to video games and is about to be adapted on an animated show as well. All of this in just 3 years of existence. Can we say that about Thunderstrike/BRB/Voodoo? Let's be real here. I'm not even saying I'm against Jane getting a new name/weapon and things going back to same old same, BUT there ARE perfectly logical reasons as to why this particular change might stick more than others.