It is a simple business decision. If your fleet is large enough for 2 types and considering that Bombardier and Embraer are gone, the only option to have some leverage with the aircraft manufacturers is to play them against each other. Which means both need to have a desire to get the deal and you must be in a position that grants both solutions equal chances.
It's interesting how much difficulty people are having unpacking what is being said.
We have 3 reasons he's raising as to why IAG signed the LOI:
IAG SA Chief Executive Officer Willie Walsh said frustration with Airbus SE over late jetliner deliveries (1) was a factor in his decision to place a $24 billion order for Boeing Co.’s grounded 737 Max model.
Cost (2) and a desire to have a mixed narrow-body fleet (3) weren’t the only considerations in the purchase, with IAG experiencing a 70-day delay on average for handovers of the A320neo aircraft, which competes with the Max, the CEO said in an interview.
If you can't take in (1), he gives you some help in taking it in:
The outline deal for 200 737s, revealed at the Paris Air Show last month, “should be an indication not just to Airbus but to everybody that we’re unhappy with their performance,” Walsh said in Brussels.
If you can't take in (3), he gives you some help in taking it in:
Walsh said he fully intends to sign off on the deal and won’t be approaching the European manufacturer, adding that he doesn’t want to be “solely dependent” on one company for his group’s entire narrow-body fleet.
It seems everyone is having no problem taking in (2).
I think the fact that he's willing to take in the 737 fully aware that it is at a very low point in its history should be read as a statement of how "unhappy" WW and in turn IAG is with the incumbent's performance.
The fact he "won’t be approaching the European manufacturer" tells you it's not about getting the lowest cost, it's about him reaching the conclusion that the only way to keep Airbus keen is to bring in a competitor.