SEN's competitive advantages were availability of slots in the London area, low charges (presumably), and a slightly different catchment area (although coastal airports in the UK have often suffered from having half their catchment area covered by sea).
Of these, the first has disappeared for the time being due to Covid.
Low charges will in future be more widely available as airports start to try and build their pax numbers again.
With loads as low as they are at present, the principal airports with the richest catchment areas will win out.
In short, it's hard to see what would attract operators to SEN. All the other LON area airports have available capacity, and airlines are likely to place their fleet where demand is high, yield is good and slots are available. That to me looks like LHR and LGW, at least for the next year or two.