When an airline has 400+ aircraft, it is probably in its interest to have aircraft from more than one manufacturer.
Boeing will believe that any order for new aircraft is a competitive bid only if they think FR genuinely would buy from Airbus. If FR somehow always end up buying Boeing, then Boeing may no longer believe the RFP is competitive and Boeing may wish to be less generous on aircraft price. An order for 50+ 737s is most definitely worth having, but unlike 2009, the economy globally is strong enough that it won't cause immeasurable pain to Boeing if they lose it to Airbus