There's a difference between "I don't want the disease because I don't want to become a menace to others" (what you're saying) and "I don't want the disease because it would make me lose social status" (what the original commenter said).
For myself: I hope for assisted suicide before Alzheimer’s. I value me for me. Not-me I don’t value, and Alzheimer’s does not improve not-me over me. But people who cannot separate me from not-me (with whom not-me loses status for me)… I don’t care about them! (Philosophical mood.)
IMHO it is important to see which context people are coming from. Different culture, social acceptance, historic treatment options, etc. influence the response.
Also, dementia is not the only disease with a strong degenerative impact on character and behaviour e.g there is schizophrenia et al. What is common with many of these conditions that there is a strong individual component of progression and morbidity.
It is great that you have the option to make a choice for yourself. Others might decide to make other choices.
I don’t want to go that way either. If I start losing my mind to Alzheimer’s or dementia I don’t want to slowly turn unrecognizable to those who love me, fuck that shit. Give me something suitable and I’ll do it my damn self if needs be.
I know there are medical directives that can be put in place but they don't cover everything and they can't compel anyone to end my hypothetical misery, the most they can do is withdraw care.