You've not just got to look at what proportion of traffic an airline operates at an airport, but what proportion of an airlines operation that airport represents i,e. SOU is more important to Loganair as it's a bigger part of their network than SOU is to EZY, so they are more likely to maintain a long term consistent schedule.
Ideally an airport wants a good mix of airlines and routes serving different sectors of the market. While EZY's presence at SOU is a good thing, the choice of routes isn't necessarily. Given the approach taken by EZY so far if you were a small airline, you would think twice about launching a new route from SOU for fear that EZY would jump on it if it was successful - MAN would seem to be the obvious candidate here.
The problem then is you could end up with losing your smaller players and have flights which are less than ideal in terms of timing and frequency and the possibility that EZY will switch out resources if they can be deployed more profitably elsewhere.