Let's be honest, Fantastic Four isn't the most popular comic.
it doesn't have to be, because the FF are iconic. Marvel's bread and butter (in terms of comic fans) are people who have been reading about these characters in one form or another for the last 20-40 years, give or take a decade.
Books go through cycles of being popular, then not, as artists and creators switch in and out and new ideas get tried out. Every character you can think of has had high and low points, has had multiple books on the shelves at once then suddenly nothing of note for months before being relaunched into the next big thing.
But marvel will always bring back their staples, because at the end of the day that's what their fans will demand. Even if FF was only selling 5K copies a month, if they canned it there would be a vocal "bring back the FF" movement. That's just the way it is.
And while it's true that the books have taken a backseat to the movies in terms of revenue, it's the fans of the books that have consistently been the most vocal evangelists of the company, and you CANNOT buy that kind of loyal fanbase, though plenty of companies have tried to astroturf their way there *cough*microsoft*cough*.
It's worth 10 times what the books bring in, in terms of goodwill and positive word of mouth. these are the people that are buying all that merchandising, and turning comic-con into a cultural event.
The FF will be fine.
