The Retribution is the carrier of Earth's retribution against the Martian aggressors when they launched a counter-offensive against the Settlement Defence Front.
The UNSA is doomed. In taking the Olympus Mons to Mars and losing both it and the Retribution in a raid on the SDF shipyard, Reyes has effectively doomed the UNSA. Despite crippling the SDF's ability to make ships, the UNSA is still left with only a single Destroyer in their "fleet," while the SDF still has ships active that were guarding targets of opportunity. With the AATIS down as well, SDF could easily wipe out the UNSA if they committed all of their remaining ships to the battle. Reyes should have kept the Olympus Mons and Retribution in a defensive role until the Earth fleets were rebuilt. While this would give SDF time to rebuild as well, it is made clear that UNSA ships are far superior to SDF ships, and the Mons would have been a game changer with other ships to keep the enemy destroyers away from it. Perhaps if Admiral Raines had survived, he could have prevented Reyes from engaging in such a foolish mission.
This does depend in large measure in how the game is played. If the player completes all targets of opportunity rather than simply the main story missions, then it's entirely plausible that both navies have been completely destroyed. While this would suggest an inability for either side to do anything, the SDF also loses its shipyard, while Earth presumably retains its yards (they're never stated to be damaged) as well as a vital resource transit hub on the Moon (well, most of it). While it would be touch-and-go, the UNSA could emerge victorious in this situation if they move quickly enough.
If you complete all of the side missions, not only is the SDF fleet left in ruins with its leadership wiped out, but the USNA also protects vital orbital infrastructure over Earth and gains control of a substantial stockpile of salvage material that the SDF recently controlled, to include a scrapyard in the outer system and an abandoned SDF destroyer.
And let's not forget that, in addition to the shipyard, Operation Burnwater has you destroy the SDF'S main fuel supply, and their main surviving refuelling station is taken by the UNSA soon after if you play Operation Sudden Death. So even if the SDF navy isn't crippled yet, they may not be able to keep going for much longer.
Though good points are raised above, it appears to have been forgotten that we are actually shown that the targets of opportunity do not represent the entirety of the SDF fleet: A large number of SDF Destroyers are present in Mars orbit and were not all wiped out in the battle with "Olympus Mons" and "Retribution." Moreover, the one taken by Salt at the end would not be able to fend off all that survived the battle. Those remaining ships could easily jump to Earth, level all military facilities from orbit, then take all needed fuel sources effectively unopposed. The SDF has a devastating advantage - the UNSA's only hope is that the remaining leadership is completely inept.
Nick Reyes has killed the highest number of people among main protagonists of Call of Duty. When the Retribution's crew took over the Olympus Mons, they went to Mars, where Reyes personally activated the F-Spar laser cannon, and destroyed several ships with it, easily killing thousands of people.
Depending on player actions, it can be even worse: factor in all the strike vessels and capital ships Reyes potentially took out during secondary missions, not to mention damage to colonies during ground ops. Reyes is easily the walking-away leader in CoD body count.
There's also the consideration that with all targets of opportunity destroyed along with the shipyard, you have likely trapped what is left of the SDF on Mars with no resources and no way out, doomed to die a slow, cold, death in the harsh weather and terrain. Even in death, his kill count increases.
All of the SDF ships destroyed in orbit over Mars by Reyes and Salter are also liable to leave orbit, especially with many of them performing combat maneuvering while Olympus Mons was tearing them apart. Whether they leave orbit by escaping the gravity well of Mars or by falling into it may also have an effect on the folks living on Mars. Earth has a similar problem to deal with due to the wreckage of the battles in orbit that took place in "Black Sky" and "Operation: Safe Harbor."
The entire plot of the game is implied to take place within a single day, more or less in real time. Which begs the question of how Retribution's crew was able to accomplish so much in repairing her within a few hours from the chaotic state we first find her in after her collision with Olympus Mons.
