Just to ease everyone's mind:
Alzheimer's disease has a totally different disease pathology than MS cognitive problems. They are two totally separate things. Cognitive problems associated with MS will NOT directly lead to Alzheimers in any way. Nor is there any indication whatsoever that MS cognitive problems will ever get as bad as "dementia".
That is not to say that someone may not come down with Alzheimers along WITH also having MS, but the two diseases are totally separate from each other.
I hope this eases your mind.