Benzos cause dementia? who knows ... but on the other side... benzos are pretty damn safe, cause they've been around for many years. and from my personal experience, I take benzos regularly for years now, got a good memory and pretty decent conversation-skills, I read stuff, learn new things and memorize with ease. My memory was much worse when I smoked ganja on a regular basis!
Even when I drank heavily, I was able to improve my skills on calculating with a training program, so the learning abilities are maybe deminished, but not destroyed. and since I don't drink any booz no more (and use no thc no more), my memory is even more accurate and reliable. So I would not worry too much. maybe just kick em off in a year or in several months to keep safe.