Well, if there were only one timeline, then they wouldn't remember their own history after erasing it. They would just fade away like Marty McFly. But most time travelers in Trek often remember their own timeline, and their timeline's history affects their new timeline (like Nero and Spock meeting young Kirk, or Ambassador Spock meeting Yar's half-Romulan daughter, or Admiral Janeway meeting her younger crew -- these remnants from the "original" timeline continue to exist and persist for a time, so even if they can't get back home, their memories of their original timeline still exists).So Nero's, Yar's and Admiral Janeway's timelines are continuing on in an alternate timeline, just without them involved. If all time travel achieves is the creation of divergent timelines, then what's the point? You don't save your own timeline, you just interfere in another!
But alternate timelines cannot "originate" from the past. Remember in "Back to the Future," when Doc Brown's dog, Einstein, takes the first trip in the DeLorean, one minute into the future. The car and dog disappear for one minute, then reappear. That's not an alternate timeline, that's just time passing normally while the time machine was taking a shortcut.That's why the events of Yesterday's Enterprise giving rise to a divergent timeline have always been as oddball to me as Time's Arrow seems to be to you. In YE there's no time meddler from the future; there's no weird time experiment being conducted in the present; Enterprise-D just happens on a timey-whimey phenomenon which it turns out originated in the past.
Likewise, when the Enterprise-C disappeared into a rift 20 years into the future as a result of its battle with the Romulans, everything that happened after that was the normal, original course of history after the Enterprise-C vanished -- including the Klingon war. When Yar decided to take the Enterprise-C backwards in time and intentionally change the natural course of history that she already knew, it was only then that the alternate timeline was created.
This is my point also. I like any time travel story, using any of those models, but my main problem with "Star Trek," given the hundreds of writers who have worked on the series, is that there are no series-wide "rules" for time travel. One week there's a predestination paradox, then another week there's "Back to the Future"-style grandfather paradoxes within a "single" timeline, and then there's the branching alternate timelines, or some combination of all of them. That's why we have these debates -- trying to retroactively impose rules on a series that never had consistent rules.Taken at face value time travel in Trek is extremely inconsistent - sometimes its the single timeline, sometimes the multiple divergent timeline, sometimes the alternate universe, sometimes the predestination paradox, sometimes something else altogether (I still have headaches about Tomorrow Is Yesterday!).
We could have the exact same debate about the "Terminator" movies (and TV series). The original "Terminator" was a self-contained predestination paradox, where Skynet created a time machine to prevent its enemy's birth, but that only allowed Connor's father to go back in time to make it happen in the first place. (Like all predestination paradoxes, the story relies on the participants' complete ignorance and incompetence in order to stumble blindly into fulfilling their own destinies.) But all the other "Terminator" sequels doubled down on the alternate timeline model, with the last one going back even decades before the paradox in the first film, creating all sorts of new paradoxes that don't make any sense -- like "Star Trek," just making up new rules with each new installment.
Simon Pegg's opinion is not canon. Okuda's encyclopedia is not canon. Their opinions about "Star Trek" canon are no more valid than yours or mine. (However, they do have the opportunity to make their opinions canon by including them in dialogue in a script, but until they do, they're still just their opinions, which I am free to disagree with.)Sweet shit! The timeline was different even before Nero's arrival. Simon Pegg said so. The upcoming officially licensed Encyclopedia says so. We don't need such TLDR posts debating the matter.