I think that reason is more prosaic. SE is interested to motivate players to play as long as possible. And they should have what to do. So, ARR crafters were motivated to levelup 8-10 classes due to crossklass skills. Single 50 class in ARR could miss crosskil and became unable to craft star items. HW added some crosskills, so omnicraft is not dead. Soul system has some motivation elements. If u want to change soul - it costs 480 reds. A lot of gametime.