Third party ownership is complicated but in principle and in practice it is wrong and must be snuffed out.
It does have echoes of slavery no matter how you look at it. Especially when you consider that, by their very nature, these arrangements are instigated with young players who have future earnings potential.
An 18 year old who desperately wants to get over that final hurdle and become a first team regular after years of training is not going to want to rock the boat. If an influential agent or other figure they trust tells them to "sign this, it'll get you some easy money up front and it'll be good for your career", they will do it and they're stuck with the consequences.