Southside you're right to an extent but a lot businesses including airports do plateau either though constraints (SEN probably fits well into that category) or because their market has a natural limit. The issue for Stobart is whether SEN's current output and actual potential is profitable, or better still provides an acceptable return on investment. Of course Stobart as an airport operator are small, it might be that their profit in SEN comes from its sales value. Or that they can grow as a business and benefit from economies of scale and relationships with airlines across a portfolio of airports.
Of course passengers is just one revenue stream, bizjets and freight for example might be a source of growth as they get squeezed out elsewhere.