May I ask why? I work for a regional "training airline" if you will. Things are far from perfect, but the vast majority of those that leave within their bond period do so because they want to fly a shiny jet, or go long haul to explore the world. That's fair, each to their own, we all have different goals in life. If everyone that left within the first year paid no bond back, the airline would be paying £15k training costs, plus 1 year salary, to only get 1 year of service out of them. That is unsustainable long term. That money could be used to increase wages for current pilots.
This isn't a question of the airline using a bond to keep T&C's low, or make money from new pilots. The people leaving aren't doing so because another operator is offering better pay and conditions, they are leaving because they had no intention of staying in the first place. I don't know any figures for who ends up paying and who doesn't, but everyone I know who has left has dutifully paid their bond, factoring that cost into their career decision. Any quite frankly they are happy (happy enough anyway) to do so, having signed the bond and been given their first airline job in the process.
Are you saying that pilots who are happy where they are should be the ones paying the price for those who break a legal agreement? Because I'll say it again - if everyone that left in the first 3 years didn't pay back their bond, the airline has 2 options. Ask for training fees up front, or go bust. There is no other sustainable option.