I think Cardiff fans should be thankful that no backfilling has been done yet. If you look at SOU in particular there has been far too much backfilling by very weak airlines (Eastern) and those spread too thinly (Loganair). They're even competing on thin routes like SOU-NCL-ABZ. Eastern were at risk of going under recently. I would say radical expansion in the face of coronavirus is tantamount to suicide. You also certainly don't want Stobart anywhere near you at the moment given their reputation and risk. They only care about one thing - Southend airport.
Better to wait it out under the radar until 2021 when things should pick up again.