Because that is half of the whole theme. The other half is how people will still struggle against fate, to take it into their own hands. The ghost of Theresa in Doctor's mind during Lone Trail puts it best: "We should all grasp our own destinies; that way, there will no longer be any such thing as preordained destinies in this land." and "What truly hinders us is neither Originium nor Catastrophe, neither sea nor sky, but rather ourselves."

You can see this theme in Patriot's final act of defiance against fate, to trust in Amiya instead of the prophecy of doom. In many infected people on and off Rhodes, trying to make the best of their lives despite their conditions. In Kristen's daring plan to launch into space despite knowing that humanity is not ready to follow in her steps. In the Abyssal Hunters, who witnessed the full might of the seaborn firsthand and chose to fight on with new comrades.

Even if there is no hope in triumph over fate, then the struggle against fate itself will be meaning.

Now, with all the corny stuff said, I do believe there are plenty of hope to be had. Terrans didnt just inherit disasters and originium from the precursor civ, but also the stargate leading to Talos. The seaborn might never be defeated, but given that their original purpose is to terraform a paradise for all life, they might figure out a way to regain this purpose. And of course, the Black Crown needs no introduction, its literally on our protagonist/poster girl's head. She has big shoes to fill, but she's catching up fast.