I guess it's a moot point, really. After all, Scott doesn't always reveal what's going on or what his intentions are. He could declare anything. At this stage, I take most of what he says as an indication of what might be happening - not as some form of Gospel.
He's got a long history of changing his mind, not delivering what we expected and in the end derailing expectations. He's a sly old fox and we should be prepared for anything.
Personally, I enjoy this aspect of him keeping us on our toes. Still, I wouldn't be surprised if this does turn out to be a trilogy. There's too many other series out there now for Fox to not want to take advantage of this current trend in movie making.
-Windebieste.