In the countries where euthanasia is commonly practiced, there are huge ethical discussions about it and the more you learn about it, the more complex it gets. It turns out, that if you get dementia, your personality changes. The new person you may become may not want to die, even if they cannot judge the situation adequately. In any medical situation a no means more than a yes and, in general, that is a very good thing.
I share your opinion that I would rather be euthanized than have my loved ones go through the whole process. But it really isn’t simple.