It is inevitable that there will be a long climb back required for the airline industry post COVID-19. But what is the source for this "best estimate" that business will only recover to 20-30% of prior levels? Perhaps the immediate restart period will see those levels, as operators will have few advance bookings at the point of relaunch. But I would expect substantial growth from there quite quickly. Not to pre C-19 levels, but certainly to well beyond 30% of previous capacity.
In terms of speculating on fleet size for individual carriers, I would suggest it is way too early for this. We need to know the duration of the crisis, which carriers receive state support, and which fail. We need to know where markets are left unserved. For the carriers which emerge from this: CRISIS = OPPORTUNITY (as the Chinese say). If several established names fall by the wayside, the survivors will have some lucrative market gaps to expand into. A land-grab opportunity will arise. Slots at previously constrained major airports. Fleet strength and deployment will depend on which carriers remain to take advantage of the situation. Let's hope that EASYJET will be amongst this fortunate group.