I don’t think it’s any kind of choice between oldies and current cast. I would also thing most of those coming back are doing cameos.
If they leave, Levi would be home alone. Maybe he would move in with Freya and the house would be let to oldies or new generation from oldies as a way to show life still goes on?
It does seem odd for them to be leaving so close to the end but it’s possible the decision was made before the contract news.
They should start a new neighbours in America with the amount of characters out there!