I'm going to inject a bit of black humour here.....True. If they're not yammering and flapping their gums they may have to start thinking. That means they'll be getting ideas and that's even more bloody dangerous
My wife has suffered from a lesser-known branch of dementia called NPH (Normal Pressure Hydrocephalus) for a number of years. As her health degrades certain features become highlighted.
The latest is that she can't think and do something else at the same time. For example, if she's walking she has to stop, think and then carry on when that thought has passed. Or, if I'm unlucky, she'll talk about that thought which normal constitutes her standing like a bloody statue for as long has she articulates that thought.
Most of the time it's funny but sometimes she stands in doorways and becomes a mobile chicane, which means if I need to make a quick dash to the bog things could become difficult