Exactly. It's one of the events (see Expressflight's post above) that can intrude when trying to forecast airport passenger numbers years in advance, along with other negative influences such as high oil prices and terrorism, or positive factors such as the startling rise of low-cost airlines during the first decade of this century that have revolutionised some regional airports, and which few if any people would have forecast in the early 1990s. Because no-one knows if or when these events will occur, or what they will be if they do, it makes accurately forecasting/predicting next to impossible most of the time.
A better guide might be to look at how an airport has performed in relation to all the others in the country because major events usually affect all airports to a greater or lesser degree.
In SOU's case it's seen a reasonably consistent performance over the past 15 years.
Looking at Table 01 of the CAA stats, in 2000 SOU handled 0.5% of the total passenger numbers at all UK airports. In 2005 this had risen to 0.8% which was the same percentage in 2010. In 2015 the percentage had dropped slightly to 0.7.