Personally, I never had a problem with the bond system when it was done for a sensible amount and reduced on a monthly basis to zero. I have had 3 bonds, 2 for 2 years and one for 3 years, all with no money up front. It seemed fair that in return for the investment in me, I should commit to stay with the airline for them to get a reasonable return on that investment. Unfortunately, the advent of PTF and buying type ratings put an end to what had been regarded as reasonable quid pro quo.
Plenty of other industries have copied aviation, they just call it unpaid internship, or as I call it modern day slavery.