Protect the regional airline's routes? Would SEN have its own catchment area? If SEN management is promoting a 45 minute train connection from Liverpool St, the catchment area will include anyone in Central London who wants to travel and is considering travel through LHR , LGW, LTN and STN. So Dash 8-400s would be competing against jets on all major routes.
Perhaps the market for SEN would be all the smaller French towns which Buzz used to serve - places within 300 miles of SEN.