As much as training capacity is indeed an issue, it's not the reason why ACMI companies exist and are doing well during times of high demand. Many larger leisure and even legacy carriers consciously choose to have a fleet of however many aircraft they can run year-round (excluding some time allowance for periods of heavy maintenance which are normally scheduled for the winter) rather than have multiple idling aircraft and crews throughout the winter. For the peak summer season, they hire the likes of Avion Express and many other, much smaller companies, mostly based in Eastern Europe. The thing is, in case of a downturn the "seasonal helpers" will be the first ones to be kicked out, well before any downsizing of the carrier's own fleet is on the cards.