Well if your old aircraft is phased out, what other option do you have?
Is a bond legal in Europe though, that's another question altogether. I know of many guys leaving Wizzair for the sandpit without paying their bond back.
Actually on that note, is there any aviation-savvy lawyer in Europe familiar with leaving before paying a bond back? I for one am not planning on leaving but it's good to know from a legal standpoint if ever I need it.