The MAX is selling much faster than the 787 did, and that was a fast seller. The MAX is also outselling the 737NG which should be a better yardstick.
But then there is the other yardstick. The Airbus A320neo family is outselling the MAX by some considerable margin. The curve shows Airbus neo orders through November, the year end numbers will be announced later in the week. The neo went on sale earlier and it has a better sales mix (essentially the A321 outsells the MAX9). Even as MAX orders track neo through year four of their campaigns, the head start Airbus has is telling.