Flybe went bust because its business model didn't work. By extension, if SOU's business model was dependent on Flybe and the airport is in continued trouble as a result of Flybe's failure, its own business model doesn't work. Neither are anything to do with the pandemic - Flybe was in dire straits long before anyone heard the word "Coronavirus". As if to illustrate the point, the reincarnation seems to be doing its level best to scatter itself thinly across the Great British countryside and repeat many of the mistakes!
The airport needs to find a new path which I think can only be based on a reduced passenger throughput, reduced operating costs and higher average income per passenger. Chasing volume at low fares has been the undoing of many a smaller airport (Coventry, Blackpool and now Doncaster to name examples) and there are few where it's truly sustainable.
It can be done and I do not subscribe to the view that the airport will close/disappear/go bust/be sold for building land. It has a market but that market is not the same size or shape as it was before, so it must adapt to that new landscape.