Previously there was a huge gap between the B737-200 and the B747-200 into which the B757/767 was neatly slotted.Now with greater capacity, longer range narrow bodies and much smaller wide bodies that gap has narrowed considerably.
Many airlines needed something between the 73 and 74 but many of those would now choose either a B737-900 or a B787-800. Only a few would specifically require something in the middle, however if the growth in air travel is sufficient it could pull the number up to a viable total.
With the MAX debacle necessitating a new narrow body sooner, rather than later and ongoing problems with the B787 and B777W Boeing won’t have the resources to take on another new project for a number of years, leaving Airbus A321 variants virtually unchallenged in this area.