Despite being a the most loyal Boeing US legacy carrier customer, clearly they could not wait any longer to replace their aging trans-Atlantic 757 fleet. I'm sure Airbus came in with a great deal to land United (assume that's why pmUA went with the aging A320/A319s - a great deal and right fit for the time).
This MAX fiasco is one big black hole. Just another wake-up call that Boeing needs a complete shake up.