The consciousness living in my body at that point may very well be happy, but that consciousness would not be me. I'd be gone, except perhaps in rare instances of lucidity.
Sure, there is a Ship of Theseus argument to be had here, but I am very much opposed to being a burden on my loved ones, and if nothing were to change with my mind, that opposition would remain. I've seen first-hand the suffering that Alzheimer's inflicts on loved ones, and that's not something I would EVER consent to while in my right mind. The very idea horrifies me.
That the alien consciousness that would be inhabiting my body at that point doesn't want to die, and holds a few of my memories, is of little consequence to me. Even if this experience is pleasant, at that point, it's not my experience -- what little of "me" is left to experience it -- that I care about.
