I have AD and I know that it is incurable and will be fatal. When I was diagnosed, over 3 years ago, I wasn't much affected by it, and it was discovered only because it interacted with drugs I was given for something else. I investigated the disease and quickly found out that it would be fatal, so I've just tried to keep fit and keep my brain busy.
It sounds as though your wife may be further on than I am, but I don't think that is the main issue. My guess is that only you can judge whether it's best to tell her or not. I see that you are trying to protect her and that is very understandable. But it will probably become more difficult to keep up the pretence as time goes by. She might find out from someone else, or in some other way. I think I would talk to her doctor and perhaps ask him or her to discuss the issue with both of you together.
I know it would be difficult to tell her, but keeping up an untruth with someone you love is surely very hard indeed. If she found out the truth it might be very difficult.
That's just my opinion, of course. Best wishes to you, whatever you decide.