There has been some progress here.
An Event ended trigger for Workflows has now been merged:
https://github.com/discourse/discourse/pull/42634
Also, Workflows already has a Topic action which can Get an existing topic and Create a new one, and there is a Wait node. So quite a lot of the proposed rollover may already be composable as a workflow rather than requiring a dedicated event-specific archiving feature.
Something along the lines of:
Event ended → Wait → Topic / Get → Topic / Create
could potentially create a successor topic using the old topic’s title/body/category data, while leaving the completed topic intact as the archive.
What would still need checking is exactly which parts of the old topic should be copied automatically - for example tags, uploads, event recurrence state, and whether replies should be moved or simply left behind.
I also have an open PR introducing an Event action to Workflows:
https://github.com/discourse/discourse/pull/42932
The base PR adds Close event and Open event, and I have a stacked follow-up adding Set attendance:
https://github.com/discourse/discourse/pull/43028
The action is structured so further event-specific operations could be added as separate follow-ups where they make sense.