I say that it's a fact our preferences are barely related to our motivations because it's trivial to show that they function independently -- you've pointed this out yourself. That most people fail to change their motivation by modifying their preferences is more than sufficient to demonstrate the lack of connection in practice between these two brain functions.

Being separate is far from the same thing as being independent, or having no connection with each other. It is only grounds for introducing a concept, for making a distinction.

Also, at this point we are entering a territory where our definitions are still at odds, for example I expect that the sense in which I talk about preferences being modified is significantly different from what you mean by that. The place of that discussion is in this linked thread.