Both my father and my father-in-law have dementia.
My father's dementia is fairly advanced (he doesn't know who I am and hasn't for a while, there are times where he doesn't know who my mother is) although he's still living at home.
My father-in-law's dementia is less advanced although he's much more frail as he was diagnosed with bladder cancer, had it removed, so has ended up with the whole tubes+bags "solution".
My OH and I were discussing this for the Nth time last week.
Apologies for being blunt, but our current perspective is this: try make your life count, and hope for it to end cleanly. Via Dignitas, if necessary.
There is no way I want to see out my final years in the way that my father or my father-in-law are doing now :(
I'm old enough to see some the advances in medical care different family members were able to receive when they became very ill over the decades. I'm young enough to be optimistic that when it's my turn the treatments available will be even more significant.
There's a certain quality of life I wouldn't want to spend a prolonged time in. I understand your point about not wanting to be in bad health. Most of the activities I do that will likely prolong my life (diet, exercise, manage stress, build social bonds, supplements, pharmacology, screenings) will also increase my likelihood of prolonging the amount of time I have in good health.
Why am I interested in living longer? I think it's better than the alternative.
I mean, why? Longer lives are not necessarily any happier. In fact, old age seems pretty physically miserable.