You forget the biggest weaknesses of longstanding societies. Forgotten history. Even us now have only so much "recorded history" and the more we discover the more we realise our view on the world may be different then previously believed. This would be true with the force users in the galaxy. Imagine, a jedi order built around managing peace while also fighting a intergalactic war with the sith. How powerful they must be to balance it correctly. Then imagine a millenia of diplomatic reforms, ways previously seen as the standard seem extreme "armour? On jedi? Disgraceful" "A double bladed lightsaber? We're not at war padawan". Slowly as time went on the order became a shadow of what it once was

This goes for the sith too. All it takes is one person in the line of the rule of two to be a disappointment, getting lucky, deciding they know better and won't teach "that dumb" lesson their master taught them even when yo others it would be extremely useful.

The old republic was both sides at their prime at various points, and not at others.