The problem is the "of sound mind" bit.
It's really difficult to prove many people aren't of sound mind when to their family and the people giving them care, they're not of sound mind.
I care for someone with anorexia who swears blind she does not want to die, she's terrified of death, she wants to live but she does not want to eat. She's been sectioned a small handful of times because she's been assessed to not be of sound mind and also a risk to herself, and then she's also avoided any sort of treatment the rest of the time even during a section because she seems to be of sound mind when a mind that is cognitively aware that not eating = dying and they continue to not eat even though they dont want to die is not sound at all.
If you've any experience with disabled adults that are disabled enough to get into an assisted living placement, and if you've ever seen how that makes them feel, you'll know they're also likely to pass gillick competency assessments, but are vulnerable to external pressures as their living situation is often out of their control and life feels hopeless when better social care could improve their lives drastically.
There are a lot of adults that fall into this category of being mentally vulnerable and coercion can be really subtle. It can be relatives sighing and huffing and puffing when they have to do essential care for you and it makes you feel worthless, to knowing you've always had support to make the big decisions in life, and you ask for advice from your support people because you trust them and they talk you round to it.
I would hate for someone to watch their family members suffer a prolonged death but I'd also hate for someone to lose a family member because they were left in an establishment that they should be able to trust has their relatives best interests at heart and that has ultimately lead to AD instead of improved care outcomes.