There's always reaction. The effect of J2's decision remains to be seen. First and foremost, how will FR react to an ambitious competitor opening a new base at 'their' airport? How will EZY respond if a price war kicks off? What about other operators?
Can't see EZY deciding they don't want any part of the increased competition, but I don't see anything negative for SEN in whatever they decide. Southend will stand or fall on it's own merits, and if EZY believes there's a good business case for a fourth airframe then one will follow, if not it wont. If, on the other hand, they're not satisfied with what their shareholders get from SEN, they'll be gone before you know it. I suspect SEN will continue to work for them in a small way for a few years yet.