Not knowing makes so many more decisions for me, however. Non-disclosure of the reality affects so much. Consider the range of decisions that might be made by one in a state of "blissful ignorance."
1) I consider this the largest: the decision to reproduce or not. First off, it runs the risk of burdening offspring with the exact same condition. Then there is the "can I even manage to raise offspring?" question, at least to the level of the average parent?
2) Similarly, seeking a partner to raise that offspring with. Imagine you had a spouse who had been kept unaware of the situation, and so you were unaware of the situation, and then you were rather suddenly burdened with taking care of someone through a prolonged decline. Would that have seemed fair to you, not to have known?
3) The rest of life planning is almost trivial next to the first couple of issues, but you think about retirement, how one's life is spent given the expected years, and so on.
Non-disclosure is a horrifying trap for more than just the patient. I would find someone who would not disclose to be inconsiderate, frankly, similar to in kind, though not in degree, as someone not warning you of a danger in a road you are about to drive down.