Earlier I replied to a post in S&T about Bionicle G1 and G2 being connected, despite Lego saying it's a reboot. That point has been refuted a thousand times, but it got me thinking about the big problem with those sorts of theories in the first place.
In science, one of the essential qualities that a theory can have is that it's falsifiable. What that means is that the theory is constructed in such a way that it can be proven demonstrably false by new evidence. Falsifiability is an essential part of science, and a big part of what separates it from pseudoscience, religion, and superstition.
Obviously Bionicle fan theories are a great deal different from scientific theories. But I can't help but think that falsifiability should still be somewhat of a priority. If nothing (not even direct and explicit contradictory statements) can disprove your theory, than how is it even worth discussing?
As an example, I can "theorize" that Matoran in G1 can reproduce like humans. Sure, Greg has stated the exact opposite numerous times. But he could have just been lying
. Unless you're willing to accept evidence at face value, you can literally claim ANYTHING about the Bionicle story or characters and never be proven wrong. Vakama never betrayed the Toa Hordika because Web of Shadows wasn't canon! Mata Nui wasn't actually a giant robot, because the events of 2008 through 2010 never actually happened! All these theories and more are valid if there is no longer a single standard of evidence that can be taken as authoritative.
P.S.: I don't want this to seem like I'm condemning headcanon. Just because the official story happens one way doesn't mean you're not allowed to reinterpret it, or pick and choose the parts you like! But for a fandom to function, there has to be some shared frame of reference that defines how a story or franchise is experienced. Headcanons won't necessarily make sense to someone who hasn't experienced the actual story and characters and doesn't understand where the headcanon diverges from that. That's the importance of canon—it defines and categorizes the official story as it was initially presented, so as to foster a shared understanding of the story among the fandom at large.