The debate and negotiations are genuine.....not simply driving the Boeing price down.
With the size of the fleet, both aircraft types (if eJ go for Airbus) would easily be big enough to offer many of the economies of scale. Once a crew / aircraft base is 'mature' then the loss of flexibility is not huge, especially when compared with the extremely competitive quotes from both suppliers for any future orders as well.
And has anyone ever considered that eJ have done a very comprehensive study.....what if they have identified the Airbus as the better of the two? Now's the time to start dealing with Airbus.
It's an open contest.