You call it abrupt, but that's actually a very typical sort of ending for a mecha anime.
It's relatively rare for a mecha anime to have the protagonist(s) involved in the geopolitics (or astropolitics) of the conflict. Usually, the protagonist of a mecha series is someone young and fairly far down the chain of command. They're junior officers, NCOs, or civilians who fell into the cockpit of an unattended robot and got drafted. They're on the sharp end of things and focused on the immediate conflict. When the final battle ends, their role in the story is over and so politicking that actually ends the conflict officially usually happens offscreen or in a quick montage that establishes peace is returning to the world.
You see this kind of quick post-conflict resolution all the time. Like in Code Geass, once Lelouch is dead Nunnally is crowned Empress offscreen and Britannia makes peace with the UFN and all is well. Or in Macross Delta, once Chancellor Brehm's plan is foiled and the Delta Wave System's shrine is damaged to the point of inoperability the Windermere forces pull out and strike a peace treaty with the New UN Gov't offscreen. Most of the major Gundam titles end this way. Once the Big Bad is dead the conflict just stops, a peace accord of some kind is reached, and the cast go their separate ways. IMO, the version that seems the most abrupt and out-of-nowhere is the one at the end of Macross II: Lovers Again, where in the space of maybe half a dozen cuts the story goes from Ingues's mobile fortress blowing up over Macross City to the up-to-that-point genocidally xenophobic Mardook having already signed a peace treaty with the UN Gov't.
Yes, but there were plans for an imminent continuation already in progress... Gundam ZZ, and technically Char's Counterattack spun off from that too.
Remember, at the time Destiny was in development and production Fukuda et. al. still thought they were going to be directly following the series up with a movie to tie up the loose ends and provide closure to the remaining characters.
But a capable creative team will nevertheless ensure that the current story has a definite ending to it in case they don't get a sequel... which paid dividends in the case of the Cosmic Era, as Destiny failed to meet Sunrise's expectations and future Cosmic Era plans were scaled back. Can you imagine how much more infuriating Destiny's kind of weak ending would be if it were also a cliffhanger for a potentially Coming Never movie?