they don't simply reset to zero, they end. that's why the Librarian wanted to send Moira off-world in Pox6; in her sixth life.
here is a map of Moira's lives, minus the sixth life. those timelines all end at a point because when Moira dies she both ends that timeline, which restarts, and forwards her consciousness to the new timeline, which she has access to in utero. those timelines no longer exist, the reality is annihilated, but reality begins again in a largely similar way to the previous reality - I can't believe I have to say this but as this is Hickman and x-men so it's probably intentional, but they're "uncannily" similar to the previous reality. the nature of her ability is reality-warping, not time manipulation, and we know this because the Librarian wanted to send her off-world to prevent her from dying when the phalanx eat the planet. the phalanx were consuming the planet for energy before assimilating the rest of the homos novissima society (see below)
...and because she ends reality when she dies, if she died when the planet was destroyed, she'd prevent the homo novissima from assimilating by wiping the board before they could take knowledge of her back to the Dominion. However, if she isn't on the "token" planet, then the homo novissima can lose that world, become assimilated into the Dominion with their knowledge of Moira, and it no longer matters if she dies or not because their knowledge transcends space-time, meaning they'll know of Moira in the next reality; and thus they could kill Moira before her X-gene manifest and the mutants lose forever.
Right now, one thing we don't know about
Moira is what exactly she has against clairvoyants, but more importantly for this discussion, how exactly do their abilities work in relation to her. My best guess? it is possible that you can see the future while (and before) Moira's alive, since reality, thus time and the future, still exist as long as she isn't killed; but that's purely speculative. according to what is shown in the text she ends reality and keeps her memories in the next reality, like some sorta destroyer class single-entity phalanx, which is why the Librarian believes the phalanx would consider her a threat if they learned of her and wouldn't abide letting a being like her exist.