Here's the Comichron sales chart's top ten for March 2018.
March 2018 Top Ten.jpg
And here's Comixology's sales chart for the week that The Mighty Thor #704 came out. Issue #705 likely did better but I forgot to save the screenshot of the sales chart for that week.
Comic Best Seller 2-21-18.PNG
Here's my point, if you love Jane Foster then don't worry too much about her being gone for very long. Real invulnerability doesn't come from being born on Krypton or eating Idunn's apples, it comes from people buying comic books. Marvel's editor in chief just "canceled" his top-selling book among women and one of his top-selling books period.
And Jane can easily be brought back as early as issue #706. All Odin has to do is say "do you want to be a Valkyrie?" In Norse legend, Odin made worthy women into Valkyrie after they'd died and there's no reason Marvel's Odin can't do that to Jane even if she doesn't want him to. I'm sure Marvel will do a way more complicated story thant that and it will be a year long crossover event that will involve every living thing in the Marvel Universe and there will hundreds of "Return of Jane Foster" comics for us to buy, hopefully all drawn by Russel Dauterman, but there's just no way Marvel will just let one of their top selling characters sit in Valhalla.
A quick point about Mjolnir and Mangog, it took the heat of a sun to forge Mjolnir so our sun likely isn't hot enough to destroy it. In issue #705, Jane threw Mangog into the sun once and he/she/it survived and came back to keep fighting. Thor Odinson hasn't shown he's worthy to weild Mjolnir yet so if Jane does stay dead and the Fenris chains give way because they can't withstand the heat of the sun, who weilds Mjolnir to fight Mangog? Maybe Thor Odinson is worthy again, maybe he's not. The two guys that need to be most concerned with Jane's death are Odin because Mangog is coming back and he has no one that he knows can weild Mjolnir and Marvel Editor In Chief, C.B. Beluski, because he has these guys he answers to called "shareholders" that might actually be more terrifying than anything in the Marvel Universe.