with (A) you don't know how far it has progressed. with (B) you know it has progressed to the point where they are mental incapable of managing their affairs. They could be a multi-millionaire and be unable to manage their finances. It has nothing to do with being poor.
I think you are confusing 'can't manage their finances' (as in does not spend money wisely) with being mentally incapable of managing finances.
You're way too smart to actually believe this. As soon as it gets diagnosed you would want to take them away. Depending on how much money they have it could be a long time before they are broke(you would want to take away control of their finances as well.)