You raise a good point, Boeing has pumped out 777-300ERs at 100 annually for quite a while and those aircraft will make up the backbone of many airline’s fleets. But many 777-300ER operators are moving to the A350-1000 to replace the 777-300ERs. The 777-8/9X are waiting for orders that may or may not show up. I would imagine a couple more airlines will opt to take the 777X eventually, but much more reluctantly compared to the 777-300ER, which was apparently the best thing since sliced bread.
The 777X is heavier than the current 777, many airlines are uncomfortable with the weight in the face of softening global air passenger and cargo demand. They don’t want to be paying for capability they may not be able to use.
I’m not saying the 777X is useless, but Boeing’s current 777 customer base is nowhere close to being “sewn up” when it comes to the 777X.
The performance difference between the 737 and A320 is very slim. Both aircraft are reliable, mature machines airlines know well. There are different operational advantages for the two aircraft and airlines have to decide which one to take based on what they care more about and what they’re willing to sacrifice. There is no clear superiority between the two aircraft, one airline will want the Airbus’ advantages and another airline will want the Boeing’s advantages. It doesn’t make one aircraft better than the other, they are just good at different things.