Speaking as a pax and interested observer I think they face some challenges in the next few years. Once the route networks have matured (there are only so many secondary or smaller less congested airports you can get to in order to do your 30 min turnaround) it will be interesting to see what the yields/load factors are on a like for like basis. My guess is that a lot of the so called growth is being hidden in the numbers with fleet expansions and new routes. Once you have a relatively fixed route network and a mature business then the challenge will be for them to improve overall profitability.
This will all need to be achieved in the face of any airport subsidies ending and the possibility of competition from other airlines and LCC's. The LCC's have been pretty smart so far at generating profits from other lines of business such as rental cars, advertising on their aircraft etc so perhaps they will transform their profit generators into other parts of the business? Be interesting to see how they fare.