Very sensibly, SilverJet are buying market share to get folks on board to spread the word. This kind of discount for the start-up period is standard practice.
My guess is that the people who will use Silver/Max/EOS are those who live closer ot them and do not want ot have to go to cronky old LHR. Although they may not get FFMs, there will be many benefits. If, say, someone is in Northampton, then LTN is well placed.
I expect that they will get some good package deals with cruise liners too bu thte dyed in the wool corporates will not move.