Star Trek: Enterprise may be the best origin story possible for a franchise the size of Star Trek. After nearly 40 years, and only ever going forward in the timeline, producers of the franchise launched Star Trek: Enterprise with the idea of establishing it as the launching point for all things Federation. The Federation of Planets, Starfleet, the alliances, and the lore of long-standing feuds were all explored in the show.
It may have had some hiccups and didn't capture the fanbase the same way that The Next Generation, Deep Space Nine, or Voyager did, but it was a show that held its own and did a lot to establish the franchise's foundation.
Sure, we had a few episodes of past Star Trek shows and even a film that dove into the beginnings of the Federation at a very primal level, but Enterprise was the concept that bound ideas together. Its legacy wasn't great then, but as time ticks by, the appreciation of the series only grows. It's a show that is gaining more and more supporters each week it seems.
Yet, the arrival of a new Star Trek "origin" story threatens that growth. A new film is coming out in the next few years that will feature the 'original-original' story of how Starfleet came to be. It'll be mostly set on Earth and likely only feature a handful of aliens, mostly the Vulcans. At least that's the working theory.
It's a concept that may be incredibly stifled by the limitations set by the extended universe. After all, you can't use a lot of aliens, a lot of concepts, or even a lot of characters. So there may be pressure to alter the established lore just to shoehorn some more established figures and aliens into the story. Doing so can and will hurt the legacy of Enterprise.
Especially if they do anything that will re-write the lore of the show, it'd be one thing to put in Jonathan Archer's father as a key figure, or even introduce a (still?) young T'Pol, but short of that, any type of toe steppin onto Enterprise will affect not only the series above but the film as well.
Fans care about the canon and they want the creators to care as much about it as the fans do.