JW411....... I think that depends, Airlines run tight margins as you know, if they lost 10% spread evenly over 1 year then maybe they would cover all flights. 10% in 1 month would be a major problem as aircraft would have to sit on the ground. 10% every year, well it depends who was leaving but if training department Pilots were leaving that fast, then that also would be a significant problem.
The other problem is resignation is contagious....