The whole "stay a year and develop" line grinds my gears, I'll bet my house that Erwin developed just as much as he did here if not better at Leeds even without playing. Add an extra £6500 a week to his pay packet and it's a no brainer for him to have moved on.
Scottish football is horrendous, he'd learn far more playing in the reserves at Leeds and on loan at League One than playing Kilmarnock and Hamilton Accies 4 times a year.