- "Shepard, thy lord and guardian, may thy assault rifle and omniblade give me strength to venture into darkness, to strike those who screweth me over down low, and to assert my dominance on all underlings who giveth me lip, for the dead may not remain dead here in Dead Space, but for you, what else is new, one of such who died to live once again? And when the dark things come out en masse, and I am surrounded by enemies, I shall smile and remember, 'I shall not go alone, for I brought the Cain,'. Boomshaklaka-ta-da!" - something that would be muttered during a final stand in Dead Space.
Isaac manages to bluff well enough, with Carver's help, to convince the masses he has a plan and to follow him out of the system in effectively a massive convoy of refugees. The co-ordinates Carver gave Isaac in that mostly on the spot speech weren't anything special just an otherwise abandoned Earthgov facility in another solar system but it's a start. A bunch of the refugees get dumped off there to hide and hopefully rebuild sooner or later.
The Bretheran Moons though aren't willing to let any part of humanity escape even if they got their new member from earth and satiated their horror hunger on the colonies. Isaac is better able to resist and see through their manipulation the second time around but it's still enough to give him massive debilitating headaches at range. But Isaac actually comes up with a plan, the mothballed but otherwise still completely serviceable planet cracker fleet.
In a twist of fate the Breatheran Moons markers enforcing that consumerist mindset to drain our planet of resources making the markers a necessity gave us exactly the tools to kill the moons. Humanity was never supposed to exist as long as it did after all nor was it supposed to develop so far. Through one means or another Isaac ends up at the controls of the Ishimura again and manages to delivery a breaking speech to the moons while they bombard his mind. Something to the effect of "you think it's okay to consume us, rip our bodies to sheds? Lets see how you like it' followed by that oh so familiar digital voice 'Initiate Planet Crack" and the psychic screams of an elderbeast as it's brainstem is ripped out by gravity tethers.
So at the end Humanity isn't an out and out victor in a case of broke your arm punching out cthulu but they're alive and the Breatheran Moons are forced into retreat after being confronted with death for the first time in eons. While Isaac started out a terrified nobody desperate to save his own ass somehow through his own cunning he managed to turn the tables on an unimaginably powerful monsterous race. Satisfaction achieved.