Well their kids could live in them, or sell it and buy a different house to live in. Forecasting what the effect of that supply will be on prices seems hard to me. Housing units per capita is lower than it was 20 years ago[0] and I think if you could find a longer time series you would see the pattern continue back many decades. That timeseries also includes all housing, not just SFH. I think if you just looked at SFH the decline would be more dramatic.