Forgetting the vertical promotion issue for a moment, command slots in mainline also have min requirements of mainline flying.
If any 30 year Dash pilots think they could go straight to a 380 command they really are dreaming (as some were back in 1992!)
They could get an F/O slot and in a couple of years a command.
Group seniority would be an overall good thing for pilots but the company only sees possible cross training costs.
