My siblings and I have been begging and pleading with them to sort this out for the past 20 years. We have offered to help. But they haven't, as said parent is "throw themselves in front of the car if we try to leave with anything" emotionally attached to this stuff and somehow maintains an inventory of everything, including how they bought it and how it made them feel.
Now they are getting old and infirm and are barely able to afford the house that they are renting, let alone put in the effort to clean it up. My siblings and I are going to be stuck with a quagmire where they only solution is to pay a professional to come over and clean out the house, and that will not be cheap.