Without slot alleviation, it'll be very hard for Norwegian to hang onto it's good slots across Europe. They may have too many aircraft by 2022
I don't think that'll be a problem, there are still plenty of airports where slots are available or even airports that aren't slot controlled at all. Of course these might not be their preferred airports, but it's better than nothing at all.
For example, let's say they are to give up their precious Gatwick slots. That doesn't have to mean they can't serve London anymore, they can still do so. Stansted still has some slots available and Southend isn't slot controlled. In fact Norwegian did fly to Stansted in the past before the moved over to Gatwick, so moving back to Stansted isn't that crazy.
Barcelona is another big Norwegian base and for now they have plenty of slots. But even if they'd lose them and can't get into Barcelona anymore, there are two alternative airports they can use. Just north of Barcelona is Girona, while at a similar distance south you find Reus. Neither of those airports is slot controlled. If Norwegian can't fly to Barcelona anymore, those are the next best thing.
Bottom line, there's always an airport they can fly to. Their network might look different, shutting down old routes and opening new ones. But they can grow back to the size they were before the Corona crisis. Just in a different shape.