When bmi regional began to restructure their costs on their services out of NEMA (and subsequently other routes) when Go began their no-frills flights from the airport in 2002, I never really figured out why bmi didn't maintain their operations at the airport.
Whilst it's clear that the no-frills airlines would take a chunk of the market, there are still those passengers that appreciate the "frills" and pay extra for this, a flexible ticket & their frequent flyer miles. regional was successful on lots of their short, business (and weekend break) routes to AMS, BRU, CDG which I think they would still be (albeit an ERJ or maybe A318/9 now the Fokkers are almost gone) very successful.
bmibaby are strongest on thier leisure routes like AGP/MJV/ALC/PMI. Whether the return of regional on the business BRU route will see other business routes going back to mainline I don't know ...