what do you mean by "increase housing stock"? create a supply of housing to meet demand for housing? i don't know much about the parisian housing situation but i really doubt more people is what paris needs. i also doubt that the sorts of houses owned by people who do not live in them will meaningfully increase the "housing stock" for the sorts of people who are trying to find a home. meanwhile, two-house owners, which is to say the rich and maybe a few businessmen who work parttime in paris, will move to strasbourg. great.
I know a guy who just found out he pays his father $1 rent each a year on six houses that he owns for tax purposes. He thinks they are all habitable properties that could be lived in. Shrug. Even if they just let a caretaker live in the house rent free to get out the 60% it's a win. If they plow the 60% into affordable housing at the expense of those who are sitting on unused housing it's a win.