You say that Ryanair would not be worried because CVT is a small airport and Jet2 a small airline?
try telling EOS, Silverjet and Maxjet that- AA are massive compared to Ryanair and in comparison to the usual airports and bases AA have STN is just as small to them as CVT is to Ryanair.
But look what happened? AA flooded the STN-NYC market with low fares just long enough to see off the aforementioned great airlines.
Dont underestimate any airline- they are all competitors and at a time when every filled seat counts, no one is beneath the radar not even Jet2.
Jet2 don't have any incomming aircraft to their fleet, so would have to have a major reshuffle to accomodate a basing at CVT or operate the previously mentioned expensive W patterns...