Nope. If you think that the FF would run-off and leave timelines/realities behind to be devoured by Galactus then you don't know the FF very well. They surely f* things up pretty regularly but they take responsibility and will put it all on the line to do the right thing.

If you're right, well then the writers have whiffed majorly.

Now, if future Franklin (Malkovich) or even a long lost Nathaniel Richards comes to the 60's FF looking for help, leaving a trail of Galactus destruction behind him then we have an interesting hump to get over.

And yes, I like the thought that they're stuck in the wrong timeline or a pocket dimension created to house them. It resolves the "are they going to back and forth every time they need to team up?" issue...their reality isn't actually theirs and/or real.