Well, sure, the book left the option open, but that doesn't mean that option has to be taken. It's not Chekhov's gun on the wall, because it's not in the first act, it's in the epilogue. It's more like the end of "Conspiracy" or "Cold Fire" -- an opening that the author leaves just in case some future author wants to pick it up, but that may never actually be followed up on. (And yes, I'm aware that both of those were followed up on in the books. Given how good the books are at picking up on dangling threads, it's hard to find one that's still dangling, at least one that was intended as setup for a sequel.)
Keep in mind that this is a collaborative and largely serial creative process, not a singular work. One author can't know what future authors might do, nor is it an author's place to try to dictate to one's successors. At best, we can offer them options.
Kirsten isn't saying there's zero chance of a reset button ever. All she's saying is that she considers it more interesting to explore the aftereffects of Janeway's death. For all she or I or anyone else knows, some editor might decide to resurrect Janeway five years from now. Or they might not. It's not all locked down in advance. Just because we aren't scrambling to push the reset button doesn't mean it doesn't exist. And just because the reset button exists doesn't mean we have to scramble to push it. Personally I hope it never gets pushed in this case; I have nothing against Janeway and frankly was kind of angry about her death, but we've had far too many resurrections in Trek and I agree with Kirsten that it's far more interesting to deal with the reality of death rather than treating it as a minor inconvenience to be erased at the first opportunity. But that's a choice, not a requirement. Someone later on might make a different choice, might come to the conclusion that there's a worthwhile story reason to bring her back in some form; but that's not a requirement either.
Peter David killed Janeway but provided an out in case it was later decided to bring her back -- in the same way that TWOK killed Spock but provided an out in case Nimoy changed his mind, and in the same way that NEM killed Data but provided an out in case Spiner changed his mind and got the opportunity to return. And in the same way that countless horror movies hint at the end that the villain might return, just in case they get to do a sequel. But that decision hasn't been made yet, and it may never be. Spock came back right away, but Data probably won't come back ever. An option is not a guarantee.
