I just had a random thought, but first... I hate 'reboots'. That's where a franchise has managed to paint itself into a corner in terms of story options, and so the owner of the franchise hires an all new production team. Out are the aging, typecast actors with their well-worn tropes, and in comes the fresh-face, young actors and new story lines that throw the book of Canon out the window.
The most recent reboot (no, not Star Wars - there's continuity there) is JJ Abrams "Star Trek". And I'm sorry, but in no way will I buy that Chris Pine's James Tiberius Kirk should command a Starship, neither by hook or by crook. In Canon, Kirk was 32 years old when he was given command of Enterprise. Young, and the youngest in Starfleet history up to that point, but there was also a clear history of performance that justified it. Kirk started at Starfleet Academy when he was 20. His excellent academic performance led to his early graduation at aged 24. For the next 8 years, Kirk served Starfleet as a fleet officer and an Academy instrutor. By the time he took command of the Enterprise, he had a record that justified it. But in JJ Abrams's universe, Kirk goes from a bar-brawling "townie" to commanding Enterprise in just four years? And he's allowed to continue to command the Enterprise by the convenience of having all other qualified officers killed or maimed while trying to stop Nero from attacking Earth to destroy the nascent Federation. Of all the people waiting in the wings, no rational organization would choose a Kirk when there was any other warm body around.
There is one bright spot to Abrams's Star Trek reboot.. continuity. Instead of a cold start retelling of the story, Abrams crafted a solution ex machina in which these events take place in an alternate past, and are guided by the The Old Series Spock. It's a clumsy and tenuous link to both allow for recylced plot lines from TOS, but also allowing for significant deviations on "what really happenned", or from Canon in general.
X-Men was my random thought. There was an evolution with the X-Men franchise starting with a series of X-Men: Origins movies which explored individual characters - chiefly Logan / Wolverine. Because he is the longest living X-Men (up until we meet Apocolypse), he was chosen to communicate with the early Charles Xavier in X-May: Days of Future Past, with the plot being to prevent the world-wide conflict between humans and mutants. The result was a complete changing of X-Men history within the context of the current timeline that also paved the way for more movies with a different cast, but allowing for continuity with the old actors and new acters as being the same individuals just in different phases of their life.
Why is this important? Because Sci-Fi franchises tend to be timeless, and as such they develop a certain 'canon' which is biblical for the fanbase. Breaking Canon is a huge no-no precisely because the fans have invested heavily in accepting that Canon - often cases contributing to it. Breaking canon means breaking your fan base, and you cannot do that except once in a generation.
Abram's other Canon-destroying effort is the new Star Wars films. Surprisingly, Star Wars Fans are not as terribly upset about Abrams's decision to violate Canon for Episodes 7-9. That's largely because the bulk of Star Wars canon was developed through novels and fan fiction that only a narrow subset of the fan base (we're talking true die-hards) are even aware of. Another important part is that Abrams is not trying to rewrite history as we've seen in X-Men, Star Trek and other franchises. Instead, he's rewriting events that haven't yet occurred, and are therefore not set in stone. As long as the flow meshes neatly with the events in episodes 4-6, there won't be a problem.
Post Script: Oh, also... most true Star Wars fans like to pretend that episodes 1-3 didn't happen, so there is total buy in on breaking Canon as needed.