It seems like if the fuel costs were to go down, you'd want a larger plane, since crew and maintenance costs would make up a larger fraction of the costs?
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Looks like the site answers this; reduced maintenance costs when not having a high-temperature turbine are predicted to lead to similar CASM as a 737 despite being about 1/3 the seats.
[edit]
Looks like the site answers this; reduced maintenance costs when not having a high-temperature turbine are predicted to lead to similar CASM as a 737 despite being about 1/3 the seats.