Something I have seen elsewhere is the implementation of “memory” type events, where after speaking to a story-telling npc, you enter an instance to complete a past world event. You would still get to play through the event as it originally occurred as if you had been there, and you could replay whenever you wish. Rewards could be removed or limited to one per account or character. Difficulty would need to be scaled in some way, as over time fewer people would be interested in running it, but it’s a good way of retaining all the great work that was put into designing these events and letting newer players experience it.