With planes, as with cars, it's the larger ones which give the manufacturer the highest profit margins.
As some have pointed out, the A321 is a lot heavier than the A320 and needs to be the size it is to maintain its operating cost per seat advantage. So an A321 shrink is not on the cards.
A 200-seat A320 stretch would seriously threaten the economic position of the A321, a situation I suspect Airbus will do everything to avoid.
The A320 NEO Plus could be an A320 stretch iso an A321 shrink, lowering operating costs per seat. The range would remain the same as current A320s because of NEO efficiency improvements.
The economic position of the A321 seems less important then the economic viability of the total A320 family. An airline selecting the 737-8 and/or 737-9 MAX because they offer the 180-200 seats better then Airbus is a bigger risk if an airline finds the 174 seat A320 has become to small and the A321 to big/ expensive. Like Ryanair and Easyjet..