Problem with dementia is the slow progression. When you can decide to end it you likely can think rationally enough that you should not and when you are at the point of it actually being bad enough you likely are not fit to decide that. Also just like a kid it can be fun to watch shows or eat tasty food or listen to or even sing songs. So if you are in a position to have a good enough environment for it it might be fine. So if your wealthy enough it might not be to bad. When you go physically down though it gets much worse as someone with physical issues can still have intellectual pursuits to enjoy. In the end the us at least does not allow any kind of death with dignity. Best you get is DNR orders and hospice which will allow you to die by not intervening. If your unconscious this can include withholding liquid till you die of thirst. No giving you to much barbituates though. That would be killing you and that would be a no no. But if you can't drink you can be allowed to die after a few days as your organs shutdown.