Who will move in and take on some of the routes?
Here is how it could work:
Routes from Norway/Sweden/Denmark to Germany/Switzerland/Austria
As far I as know there is some sort of ATI between LH and SAS on those routes between Germany and Norway/Sweden/Denmark and I think the same applies to OS and LX in relation to Austria and Switzerland
So if SAS closed down, LH/OS/LX will take control of those routes
All other Mainline Routes from Copenhagen, Oslo and Stockholm:
I will suppose overseas airline competing on overseas routes served by SAS will increase the frequencies, anything else will have to be filled by Norwegian
What to do with the remainder of SAS:
I would guess Wideroe will be either sold off or the Government of Norway will take control of the airline and the stake in Air Greenland will be sold off to the governments of Greenland and Denmark