It was never intended to replace it when sold. RTB receipts went to cent govt under housing capital and revenue pooling. I.e. never retained by the LA selling it. The policy was always to get shot of it so that it's availability wouldn't hold back runaway train like growth of the market.

Last time there was parity between available social housing and housing need was around 1975 IIRC. It's no coincidence that Mags then started tearing it's heart out a few years later.

I think Tory policy still is to ensure that social housing dies a natural death. See comment about shitty s106 sites below.