1. The minor differences in crew in YE are no more significant than the episode-to-episode differences in crew seen in the prime timeline.
2. What new technologies exist in the 2360s that don't exist in the 2340s, and that would influence the outcome of a war? Similarly, what's new in the 2370s that wouldn't already exist in the 2360s?
Military technologies can evolve significantly in a decade. The Federation, which had been prepping for war against the Borg and Dominion, had advanced its technology beyond what it had in the 2340s.
Needless to say, the advancement of technology moves much faster in a civilization preparing for war than one at war.
Notice how Klingon ships never seem to change during the Dominion War, while Starfleet's ships eventually become unrecognizable. Initially, Starfleet battlegroups consist of Mirandas and Excelsiors, but, by Chin'toka and the invasion of Cardassia, Starfleet consists of Akiras and Galaxies. Apart from that one Negh'Var in "The Way of the Warrior," we see only Birds of Prey and battleships in Klingon battlegroups.
That shows that Starfleet evolves, and comes up with more advanced ships, while the Klingons simply construct the same ships over and over again. If Starfleet evolves and the Klingons don't , how can it possibly be defeated?