Legally, it's the local authority who has a duty of care towards adults with disabilities in the UK.
Unpaid carers currently save the government nearly 2 billion in care costs annually. They don't have to do it. They can down tools at any time, and step aside, leaving the local government to step in and meet the person's needs.
So if these unpaid carers suddenly can't afford to continue in their caring role (because their loved one lost PIP and they no longer get carer's allowance) then it's going to cost the local authority lots of money. Councils will be bankrupt, but then they still have to provide essential services including social care.