I feel your pain.
My PILs were hoarders and it has taken three years from FILs death to clear out just one place and make a start on the other. That's most weekends engaged in one way or another with clearing, planning clearing, Ebay, charity shop runs, arguments about other people's old things and dump runs.
Their parents were just as bad and DH can be pretty awful, and his brother and SIL even worse. Broken crap, moth eaten things nobody has used since 1920 stored in old trunks, dangerous or uneconomic old electrical items, knackered and smelly beds, old newspapers, collections of plastic carrier bags (as in deployed as collectables). Human beings and their needs and disregarded in favour of the hoarded items, which are often poorly kept and unusable by the time they are rediscovered. For example we have just found a 1933 Raleigh bicycle in its original form that would be worth hundreds had it been put away in the adjacent garage but DH's grandad elected to put it at the back of the (half open to the elements) coal shed where it was left to rust over the subsequent forty years.
Meanwhile the house was rammed with all this stuff to the point that, on the rare occasions we did go to stay when they were still alive, we had nowhere to unpack, the house was unaired and smelled, as climbing over things to get to the windows was problematic and many of the windows had home-made double glazing on there that couldn't be removed, beds had grubby sheets and the house was glacial as the heating was practically non-existent. Very unwelcoming. Yet they genuinely wanted us to be there, oddly.
I think running houses like this is form of laziness and greed. Laziness in that you leave your sorting and disposal problem to other people ultimately, and greed in terms of taking up a bigger space footprint than others as your 'stuff' ostensibly has priority over other human beings and you are not thinking about the space other people might need or appreciate when they come to visit. I find it disrespectful of other people in both regards.
I think things like the Kondo book are valuable in that it breaks the hoarding cycle for people, and as another poster says, liberates them from their possessions and helps them form their own identity in relation to the past. Meanwhile it makes the hoarders easier to visit and spend time with as they get older (when they would have had a lifetime's hoarding under their belt).
There should be a lot more programmes and books on the difficulties of house clearances and dealing with hoarders, I think, as too many people have their heads in the sand and impose on their relatives terribly. Misery all round.