Manston isn't a fair comparison in my opinion. Southend is in a far more viable location and has a much larger local population. Planestation created a comprehensive route network from day 1 that echoed operations at other more established airports that had developed over a few years. All utilising an airline no one knew from an airport many didn't know existed, starting after the summer peak. It was always going to end in tears.
If it doesn't work I accept that credibility will be impacted, but the choice of routes is fairly safe and plays to SEN's strength which is primarily leisure. I can't see any of them being an outright disaster.