If it hadn't done well, it was a companion prequel, a one off hypothesis on how things happened. I think when they start mention reboot, it means that they hope to have sequels here and want to free themselves from having to be consistent with the originals. I think my one complaint overall about this movie is that maybe it should have been two parts to begin with- Just an Xavier and Magneto movie and then a creation and recruitment film exploring younger players. However, I don't think the producers felt they had the kind of time to do that, you know what I mean? Kind of like the first Lord of the Rings cartoon that just falls off at the end with no resolution.
I want to see a Brotherhood movie next!