This assumes a significant number of those spaces are contiguous such that they're eligible for being replaced by housing. Moreover, Paris doesn't build tall buildings, so the housing isn't very dense (compared to a typical US city). I'm very skeptical this would create enough housing to lower property values.
EDIT: Oof, that's quite an error on my part. I was absent-mindedly conflating "downtown areas" with the whole city. My bad. Never the less, I'm skeptical that this change will amount to much in terms of lower property values, but I'd be happy for someone to articulate why my skepticism is misplaced.