At most, you can end a timeline, prevent it from going forward, and erase the quantum information of it at the moment it ends, so that nobody remembers it happened. But it will still have happened, and any omniscient observer outside of time such as a Q or a Prophet, viewing the entire tree of branching timelines, will still be able to see that it's there for the duration of its existence.
The authors of Coda chose to disregard this for their own narrative purposes, and I suppose you could argue that from a narrative standpoint, erasing people's memory that a timeline ever existed is much the same thing as retroactively erasing its actual existence. But by the rules I laid out in DTI, and by simple logic and scientific sense, any event that happened did happen, even if nobody remembers it later.
Okay, I need to step in here — NO, WE DID NOT. We did
no such thing. If that's what you think we did, you’ve had a failure of reading comprehension.
At several points in the text of
Oblivion’s Gate, it is stated explicitly that certain entities, such as those you describe, DO remember the events of the First Splinter timeline: the Prophets, the Q, the Travelers, and Benny Russell (and possibly many others). In addition, the novel’s final chapter is meant to convey that echoes of the First Splinter (and many others) live on in every quantum temporal variation of Jean-Luc Picard himself, as he was at the "fulcrum point in time" when the First Splinter was undone.
Coda is not a betrayal of the First Splinter stories, it is a celebration of them, an affirmation that their existence matters because in the end they “kept the flame alive” for the Prime incarnations of Trek. No stories were destroyed or taken away from anyone — they are all still there, and you can read them anytime you want.
Oh, none of it matters because it came to an end? EVERYTHING comes to an end. Our universe's most likely conclusion is an eternity of entropic heat death. Nothing we do will matter in the scope of that. But in the struggles of
this moment, if nothing we do matters, all that matters is what we do (as they said on
Angel).
Coda is a story about the inevitablity of change and death, but it's also a tale of raging against the dying of the light, about fighting for hope and what's right even when no one will know of your sacrifice. It's about being willing to sacrifice oneself for an idea, a principle, for love, with no expectation of reward.
As far as I'm concerned, stories don't get much more
Star Trek than that.