Oneness
Luke and Vader's victory at Endor caused more harm than good.The Prophets of Rhand said it best when they proclaimed that Palpatine's failure was that, after he succeeded in destroying the Jedi and the Republic, was that he tried to create - potentially his '10,000 year reign' (most likely as a body hopper between Force sensitives) would have seen him ruling over more and more systems - but as a Roman Emperor archetype.
You know, I'm pretty sure 10,000 years under Sith would be a heck of a lot worse than the Vong war ending a bit faster.
There's constant suffering under such a system, and there's large amounts of the galaxy- the unknown region- that would have large numbers of worlds put to the torch under the conquest.
Furthermore, that assumes the rebels didn't success sooner or later anyway. It certainly would've been a longer and bloodier rebellion, but victory was not impossible. What if a ship Palpatine was on was destroyed in 18ABY? Then the fall of the Empire would still happen and there'd be less time to recover from it when the Vong did show up.
Or what if Vader dethroned Palpatine but failed to hold power due to his heavy handed methods? Or if master and apprentice mutual-killed?
The Empire inherently had a lynchpin. It was not designed as a stable system that could handle without it's leader well.
The Jedi and the Sith allow the destructive wars of Star Wars to perpetuate increasingly. They are both equally responsible for getting in the way of peace and little the normal, non-super beings who can't use the Force to govern themselves.
The Jedi only get in the way of 'peace' with it's an oppressive 'peace' that enslaves and kills countless people across the galaxy. I will again note, most of the time since their forming have been long, long periods of peace. Sometimes lasting for ten thousand years, sometimes one thousand, sometimes less, but overall, a good enough track record that I feel confident to say, 'these are not the instigators here.'
The Sith get in the way of peace because that's just what they do.
The Jedi aim for peace. The Sith aim for conflict. When the two are balanced, you don't have peace- view The Old Republic era for that, two governments, one with Sith one with Jedi, and you get destructive struggle.
Nor are lack of Jedi a sign of peace. One of the worst conflicts in galactic history, the Pius Dea Crusades, happened when the Jedi withdrew, unwilling to act, and went on for a thousand years until the Jedi allied with other resistance forces and took them down. No Sith, no force users in the helm. Just billions and billions dying for a millennium.
Furthermore, the Tion conflicts, and the time of Xim the Despot, no force users were behind that. Nor the Hutt wars. The Alsakan Conflicts were seventeen wars driven by political tensions and not force users.
The Jedi help reduce such conflicts.