If your preference is that you should have as much decision-making ability for yourself as possible, why do you think that this preference wouldn't be supported and even enhanced by an AI that was properly programmed to respect said preference?

Because it can't do two things when only one choice is possible (e.g. save my child and the 1000 other children in this artificial scenario). You can design a utility function that tries to do a minimal amount of collateral damage, but you can't make one which turns out rosy for everyone.

e.g. would you be okay with an AI that defends your decision-making ability by defending humanity against those species of mind-enslaving extraterrestrials that are about to invade us? or e.g. by curing Alzheimer's? Or e.g. by stopping that tsunami that by drowning you would have stopped you from having any further say in your future?

That would not be the full extent of its action and the end of the story. You give it absolute power and a utility function that lets it use that power, it will eventually use it in some way that someone, somewhere considers abusive.