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.
... unless the risk and cost model they're looking at is that having multiple planes in the fleet increases the risk that spare parts are depleted for a particular model, ground crews aren't sufficiently cross-trained and errors on their part make flights less safe on average than in a monoculture, etc.
It's entirely possible (and for airlines operating a wide variety of different length routes, entirely necessary) to safely operate a wide variety of airframes, but it does entail paying for a lot more redundancy.