After 15 years, a person has enough experiential wisdom to determine whether a particular art or discipline is right for him/her, and also to train in other arts without any problems. A sensei who frowns on this, seems like a controlling or jealous person who doesn't want his or her students to mature and pursue further development for their own benefit. Either that, or it's a money thing...
I dont think its a money thing at all. I think perhaps it is more of a "property" thing. I think sometimes students are seen as a commodity in the dojo and the Sensei is the owner. Or another perspective is a Father or Mother to children. I do feel there is a sense of ownership there but at what cost? Is the child supposed to become your clone? Are you to shield this child from other perspectives and views? Being a parent also I see some of the reasons why this may be happening but what is different is I am an adult. I will always be grateful for my time and training with my sensei but, the world is bigger than this organization or dojo.