It's a catch 22 for the airlines, if they don't sell future flights then there won't be any passengers on them when they take off but if they do sell tickets the flights may not be allowed to operate. Whilst cash flow needs to be maintained, it shouldn't be at passengers expense. A flight credit is of very little use to someone who needed to travel last week for a specific reason which no longer applies.
Airlines need to guarantee an immediate refund back to the passengers credit card as soon as a cancellation takes place. There are people trying to get home who have endured multiple cancellations and whose credit cards are now maxed out until their money is returned, or who have been offered rebooking within 12 months when they will probably be unable to go anywhere.
Hub airlines will likely suffer more in the aftermath as passengers will want a direct flight between two relatively unaffected countries rather than a transit where they mix with people from all over the world. Countries will recover at different times so for example in six months Australia and the UK may be clear but India and Africa are still badly affected. Emirates may have to fly Sydney - London via Dubai but with the stop in Dubai simply to refuel and change crew, all pax remain onboard and no new pax join the flight. This may build up to limited transits being allowed, but the major crossroads of pax from all over the mixing together will be out for a long time to come.