Long ago I came to the conclusion that the only way to satisfy airline requirements would be to desing an aircraft which can be streteched or shrunk in increment/decrements of 1 frame, with 5 different wing & empenage configurations and then to have a range of three different aircraft single-aisle narrow-boddy, double-aisle mid-size sort-of-wideboy and "multiple-aisle or multiple deck" extra widebody. And you apply the '1-frame and 5 wing/emp. config' to each set of aircraft.
Then you sell the aircraft for 1 sector per day & buy it back on arrival and sell a replacement which is EXACTLY what is needed for the subsequent sector.
So on monday morning you sell a great big "extra widebody" for LHR-JFK and buy it back in JFK & sell a "mid-widybody" for the return JFK-LHR.
And even then they'd complain that they weren't getting propper utilisation of the aircraft...