@Green Witch,
Of course they will.
SOMEONE will fill the demand for vehicles. It will be the companies who produce what the consumers want.
GM dealers will convert to Honda dealers.
GM parts suppliers will convert to Toyota parts suppliers (if they haven't already, I'm quite sure most already do this).
GM auto mechanics will still have GM vehicles to work on for the next 20-30 years, not a many, so they'll supplement with working on Nissans.
Something/someone will fill the demands.