...the Empire still had it's massive navy, and various admirals, and the bulk of it's conquered worlds.
Wasn't blowing up the DSII=game over kind of dumb?
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That's covered in both Legends and Canon. Basically, Palpatine deliberately set up the Empire to not have a proper chain of command with clear lines of succession. At the same time, a vast majority of the Empire's higher ranked military figures were at Endor, most stationed on the Executor. With the Emperor, Vader, and most of the Grand Admirals all wiped out in one battle, suddenly there was no clear leadership. Which meant no coordination between the very spread-thin Empire. So when countless worlds took the opportunity to rise up against their oppressors, the Empire simply didn't have a coordinated response against that sudden outpouring of scattered rebellion.
And even then, the fight wasn't over. The DSII being destroyed didn't end the Galactic Civil War. In Legends, it raged on for another decade and a half. In Canon, we still had Palpatine's Operation Cinder which went on for another year and was deliberately designed to spitefully burn the galaxy down after his death, targetting both Rebellion worlds and worlds sympathetic to the Empire.
So, yeah. You're right that blowing up the DSII didn't equal game over. Because it wasn't game over. The war kept going, no matter which continuity you look at. But the destruction of the DSII was the major death knell foretelling the Empire's downfall. Without its central leadership, the Empire would inevitably collapse due to the rampant infighting resulting from the power vacuum. Many Admirals and Governors decided to simply try and hold on to what pocket of space was already under their control, and refused to share resources with other Imperial figures vying for power. Meanwhile, the Rebellion would start to systematically wipe out those Admirals and Governors, growing the New Republic's forces day by day with each planet they liberated. With the Empire in disarray and the Rebellion forming a coordinated effrot to liberate any worlds resisting the Empire's occupation, there was simply no hope of recovery for the Empire.
I'm glad it ended rather quickly in canon, tbh... Especially since Thrawn was already out of the picture by then.
It was basically:
Palpatine "dies" > Operation Cinder commences > a few more major battles between the Empire/Rebellion take place over the course of roughly a year > the Empire is officially "defeated" > the majority of the Empire's remaining loyalists take refuge in the Unknown Regions, which ultimately leads to the formation of the First & Final Order(s).
Not all the GA died at Endor, the Empire could have still won as Pellaeon said if they had just stopped bickering. I mean GA Teshik held off the entire Rebel Fleet with just his Star Destroyer.
That's why I said "most of the grand admirals," not "all of the grand admirals." Of course a small handful survived, but that still just lead to more of the infighting and bickering that Palpatine had deliberately engineered into the Empire.
Honestly, the collapse of the Empire had way more to do with Palpatine's petty nature than any individual act of the Rebellion.