Another classic...
Why on earth should people be taxed because their kids have grown up and left home? It's what this idea amounts to most of the time.
We have the opposite at the moment; in many cases it's expensive to downsize and any surplus cash becomes taxable when you die.
The second one to die could have a large untaxable asset, sell it and you're into c40% IHT.
My mother's just had this, luckily she'd blown her surplus cash on care home fees so not too much to pay.
Must say i felt a bit guilty living in a four bed alone and looked into the options but that was solved when current OH moved in with three kids.
There should be some incentive for moving to a smaller house when the kids leave but a punitive bedroom tax isn't it.