My Dad died in a care home with vascular dementia progressed to the point where he didn't recognise us nor Mum.[Alzheimer's] lIt is not a pleasant way to go.
I doubt the assisted dying proposal would help many with dementia as (my understanding) most with dementia die of something else eg my Dad died of Covid in a care home. No "less than 6 months" prognosis.
But (as we're talking about dementia) significant is the psychological hell Dad went through when it became apparent he had dementia. Sister in law's Mum he knew had severe dementia for years so he knew exactly where he was going, what was going to happen to him and nothing on Earth can stop it.
Ian