I can't see the problem. Most, if not all major European airlines are TRTOs. They are training organisations. Their trainees arrive for their line training with X hours, at least 6 landings (normally in the real thing, but not necessarily) and a type rating. For the more nervous companies, they might even stick an alleged Safety FO in the jump-seat. They then fly with fare paying passengers. Whether or not the trainee's salary is paid by that airline is irrelevant. Generally, the most difficult thing to manage with legacy airlines is the protocol whereby the person who is replaced has to be "ghosted." The training period may last until the end of line training or even for a six months or so. What is certain is that all parties gain out of this process and the passengers never lose.