It's a fairly small sector of the market. Monthly average mortgage origination in 2016 were $158B. That article talks about $16B in MBS. That's not nothing, but

It's a fairly small sector of the market. Monthly average mortgage origination in 2016 were $158B. That article talks about $16B in MBS. That's not nothing, but it is a small section of the market. The government has dominated the mortgage market since the crash and via that indirectly exerts a lot of control over the housing market. For example, it is the GSE's requirement that mortgagees in nominally 100 year flood plains carry flood insurance and that NFIP, despite its outdated maps and low policy maximums, is sufficient. If the mortgages were mostly held by private lenders its likely they would require properly underwritten flood policies to at least the full value of the mortgage.