I think Ryanair is a threat at STN; they fly to most destinations and will compete on price. They have burnt Easy's fingers before - remember LTN-CPH.
Unless you can spot an Achilles heal and go in big (Jet2 really dramatically expanded the holiday market) STN is difficult- there are other places to go. Anyway STN is arguably doing very well, Jet2,Wow, etc. etc.