Actually it's not that unlikely. As much as Fox has been inconsistent it has to be noted they have been more hit than miss in the past years. Yes, you still have movies like Wolverine or Apocalypse or Fan4stic but they don't miss like in the early years. So if we are going by chances there is a pretty decent chance the next Fox F4 film will be solid...
But, but BUT... you're ignoring the reality of FF. Fox has destroyed the property with two bad and one
terrible film to the point fans aren't just ambivalent but violently HATE the idea of another FF film.
Last time, they didn't just lose a
lot of money at the box office (as they obviously did), but they literally lost
BILIONS (
http://forums.superherohype.com/showpost.php?p=34606055&postcount=574 ) in market cap and the chairmen specifically pointed his finger at Fantastic Four as a primary reason.
There has never, in the history of film, been a situation in which a film studio has screwed up as bad as they did with the three FF films, and they've put themselves in a position in which people are violently opposed to the idea of another film. That, in itself, makes FF a big risk. And Fox doesn't like risk. They like big bang for the buck and they are only willing to spend a little bit when they have a proven formula like X-Men.
But Fantastic Four can't be done on a shoe-string budget (as we've seen). When Fantastic Four first came out in the early '60s, it was unique because, unlike other popular comic books of the day, it was epic sci-fi with strange creatures coming from under ground, outer-space and alternate dimensions. Those spectacular situations and visuals that characterized the FF can't be done cheap.
AND because Fox has screwed up so badly and because fans hate the idea, directors and actors would be the object of hatred from fans from the moment they sign on. What talented director or actor wants their twitter feed to be filled with nothing but animosity from angry fans? What talented people would sign on for such an effort?
Add all that up and FF isn't just another X-Film and can't be treated as one.