Airlines could seek exemption from this rule if they guaranteed to have a minimum % deferred items both on individual aircraft and fleet-wide at any time, coupled with being below a minimum % cancellations declared due to maintenance, perhaps? (Subject to submission of their aircraft and maintenance records to an authorised supervisory agency).
At present in the opinion of the Court the consumer is not protected from an airline spinning them a line when they just don't want to operate the sector, but I think the ECJ ruling can be replaced with something fairer if the airlines, realising the cost implications of this ruling, come forward with a reasonable, enforceable compromise. After all, airlines seem to find ways to operate in a timely manner from airports with slot restrictions - to the point of flying empty to keep the slot!