Boeing has open production slots for the 737NG and the 777 while they update for the MAX and the 777X. Slowing production then speeding it up again is hugely expensive and disruptive - so they can effectively sell the NG and triple at cost and still be money ahead. Further, spitting out 737s at ~50 a month allows huge economies of scale that simply are not available to the C Series (C Series at ~50/month would exhaust their order base in six months
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