I think it's a great idea. It has long irked me, that when people talk about underused accommodation, they talk of council housing only.
Yes, people who have worked hard all their life should have the cushty retirement that they have earned, but I fail to see why so many want to continue to live in enormous houses. I am assuming that no-one will be forced out, just incentives, so if it's still important for someone to have a home office or spare rooms for grandkids then they can choose to use their finances for this.
I live on a mixed estate of nice bungalows, nice family houses and some cramped flats. There's also a very small amount of social housing which I live in. The bungalows are mainly appropriately used by the elderly, but the 70s style 3-bed houses also seem to be mainly occupied by 1 or 2 elderly people. The flats are lived in mainly by working families with 2/3/4 children. Lots of the adults have respectable jobs, nurses and teachers for eg. The flats aren't awful, but it seems crazy that families whose parents have taken responsible career paths cannot afford somewhere with a garden and are stuck in flats. If the children dare to play outside, they are soon reprimanded by the elderly who live in all the nice houses on the edge of the green.
And don't get me started on the area I work in. Very wealthy area where I know of virtually no young families, but elderly people living in houses with derelict tennis courts and swimming pools. Often these people don't even use the upstairs of their homes. I would much rather these sort of areas were redeveloped with useful housing rather than covering greenbelt land with more tarmac. Of course these people could demand a very high price and live in a very great amount of comfort for the rest of their days.