I thought the super duper 737 (aka 8/9/10 MAX) was supposed to be sort of a 757 killer (even though the 757 has been dead since they shutdown the production line). It carries almost as many paying passengers as the 757 but burns a lot less fuel... and sacrificing some performance that you only need to get out of certain high elevation airports.
I think the A220 sorta splits the difference of the various non-MAX 737s; the seating capacity of the older ones still in service but with range of the newer series.
Apples, apples, and more different kinds of apples.
Fuel savings is profit in good times and the difference between bankruptcy and staying afloat in bad times.