There's still hope for a shortened season, even if the GP events are cancelled. National events don't start until mid December, and only the athletes who aren't training in their country in the weeks leading up to the event would have any issues with quarantine. 4CC would probably be tougher to put on than Euros, but it's also scheduled later so it might be feasible.
Tennis will start up in the next few months, with a similar situation of athletes and their teams traveling to an event from all over the world. So I think by March/April the ISU will have several examples from other sports that can provide guidance on how to put on a major event as safely as possible.
I just had a thought. There is also the 2021 Winter Universiade to be held in Lucerne, Switzerland. It is scheduled for January and covers multiple winter sports. If this goes ahead then Euros and Worlds should be feasible.
If not...I wonder what would happen as these events take place every 2 years.