It may be an oligopoly, but it is in the airlines interests to keep them on their toes.

If you're an airline of any size it would make sense to have relationships with multiple manufacturers, for the reasons you describe.

They can decrease their Boeing orders and increase competitors orders, to maintain relationships and punish Boeing.

If you single source your your fleet, you increase the odds of something like this grounding your entire fleet, and you don't even have any leverage to negotiate better terms from Boeing.