Never a good thing when an Airports performance is so heavily dependent on one operator.
Unfortunately, unless another one acquires a big fleet of aircraft types that are compatible with the constraints on ops at SOU, and move in to compete with Flybe, that seems likely to remain the case.
The fact that Flybe re-trenched to some extent at SOU in 2015 (and opened a base at BOH) cannot have helped.
As Adfly says, Flybe are showing a significant expansion (compared to 2015) for 2016 and the Airport have obviously been working to get some other operators in, so it is looking more positive.
As a fairly regular user, SOU works great for me when flying domestically. All the upsides are landside (road, rail, parking close to Terminal etc etc) and all the downsides are airside - as documented regularly on here.