YABU.
It's all very well saying that, but life isn't always that simple.
At what point do you throw them out of the home they have decorated/furnished?
The second their oldest child goes to Uni?
After all, some children never really return home except to visit, so why should that child still have a home?
Then downgrade again if a second child leaves?
Who is paying for all these moves, what happens to all their stuff?
Why would you introduce so much instability into peoples lives when part of the intention of social housing is to provide some stability?
How about instead of them moving their parent in to care for them, the state can pay for that parent to be in a care home as they definitely won't have the space?
Or what if their uni child needs somewhere to live after uni or if it just doesn't work out?
Should they be homeless because after decades of being occupied, a room was empty for a few months?
What about the younger unmarried kids who join the military?
Don't they get homes to return to between postings?
What about the family who choose to become a foster family once their own children are grown?
Shall we just leave those kids without a home because you don't like the idea of someone having a cm more space for a bit?
What about disabled people who need an extra room for their overnight carer or equipment due to their disability?
A home isn't just about how many rooms are in it.