I’d always come back to a reasonableness test - or the what would the man on the Clapham Omnibus say test.
In ordinary circumstances, an airline cancelling flights to reduce capacity because it is undersold is clearly something in its control. It would be reasonable to expect compensation.
In today’s circumstances, yes you can argue that the airline is voluntarily cancelling some flights. But I think the reasonable man on the Clapham Omnibus would say that the circumstances which have led them to do so are exceptional, and therefore compensation should not be owed.