Where there's a multitude of heirs and an executor to deal with the estate before they can inherit, that's one kind of nightmare.
But when there's a sole heir, who will inherit the property itself and become liable for the costs until it's sold, I can see people dying.
Imagine losing your home because you can't afford your mortgage and living expenses and the running costs if the flat, so you eventually go bankrupt. But you can't get any assistance because you own property (which you can't live in).
Imagine living hand to mouth and inheriting these extra charges, going into debt instantly, ending up with bailiffs chasing you and a CCJ, even if you manage to sell it before bankruptcy your normal everyday life is still impacted going forward. Assuming the stress didn't drive you to suicide/addiction/illness.
Imagine means tested benefits being your sole income and losing your entire income overnight. How do you pay rent? Feed yourself? Not just for a few months but for years until the flat is sold. Will people end up on the street?
If people leave them to charity to avoid inconveniencing loved ones, imagine the impact on the charity. Do you really want eg Samaritans or Childline answering less phone calls from distressed desperate people because £10k a year of funds is going to the management company of the flat you left them? What about all the charities that would go bust due to these "kind" bequests? Do people realise how much social care is farmed out to charities and not provided by the state? Children's charities, mental health, domestic violence, homelessness, poverty, disability, terminal illness and elderly people's charities etc. Do people want others to go without help because charities have closed or have less funds?
These flats are a frigging disaster for everyone other than the person who lives in them. Or perhaps the well off who can afford the costs until it's sold, recouping their losses at that point. It's going to end up impacting society as a whole.