I see no reason why replacing existing housing stock with well designed mainly flatted developments should be any better than well designed mainly flatted developments on brownfield sites (and some greenfield sites too).

Most retired people, who have paid off their mortgages would prefer to keep the house they have rather than move into smaller properties, and the same is true with council tenants. But I would far prefer that the government finds solutions other than forcing retired people out of their homes to build them flats as a replacement.

Most housing growth has been in 1-2 bedroom flats, but there continues to be at least the same level of demand for family units (3+ bedrooms), which is why the council has rules about replacing demolished family housing with at least the same amount of family housing (usually in a smaller space).