Only one little problem, 777x - if the fares dropped by only 1-5%, then that means more empty seats for the airline operating A3XXs, unless the route had extremely high demand! Fares dictate load factors quite a bit. And vice versa. So, fares are not something any responsible airline would fool around with, unless they fly to places with little or no competition.
While airlines try to maximize profits whenevr they can, they still have to be careful. While it's true that lower fares may be good for the passengers and bad for the airline's revenues, airlines would risk more empty seats if the fares didn't drop by that much - and that means even less profit than an airline would make on a 747-400!
The A3XX is a bigger plane with more seats and needs more fuel, more FAs and maintenence, and that's why the airline'd risk losing more money. That's also why a 10-20% drop in fares is reasonable.
But I do find the idea about extra space for gyms funny, especially in this age of high passenger traffic!