Is hard to do with multiple writers, likely if you go the X period model the "how long till reboot" should just be tied to "how long is a given writer writing" no handing off, each one does their story. If that takes two years, great, if it takes 20 and sales let them last that long, great. (this is a thing that happens with serialized stories with beginning middle and ends, sometimes the story don't sell so it gets no ending or a rushed one, so... if your into that model something to be ready to deal with.)
As for the overall question of the thread...define Marvel universe and define shutters. Comics, even US ones are doing great. In terms of sale its 90% scholastic, but people are in fact buying comics. And that's not on top of the... less than effective at getting people paid models like webcomics. They may not make people much money but lots of them get made and read. The medium is in zero danger of going anywhere. And odds are Marvel is in no danger of going anywhere.
But the 20ish page monthly publication? Harder to say. Right now Marvel and DC and a lot of other "not aimed at school children" comic publishers are set up so they are dependent on those to fund everything. And they are at least just about still working. So transitioning away will be slow, but I do think long term likely to happen. How long? No clue. Could be centuries, could be decades or less, or more. Now the "no rally this is all a connected universe" Marvel U in its current state might be able to transition to whatever distribution model comes next, but might not. But Spider-Man comics in some form are going to be around for a lonnnnnnnnnnnng time to come. Even if they no longer are part of 616 or come out monthly.