Look, the reason people are being critical of you, BR715, is that you're basically asking an unrealistic set of questions. It's great that you'd like to start an airline in 15-20 years; just don't forget that the vast majority of airlines started in the last 20 years have all failed, and that the U.S. industry as a whole has had a net loss since deregulation. It's been said that the best way to become a multi-millionaire is to be a billionaire and start an airline.
Very few in 1982 (20 years ago) would have anticipated that Pan Am, TWA, and Eastern would no longer be flying in 20 years (and very few would have predicted the demise of Braniff a few years earlier). Very few would have imagined that tiny little Southwest Airlines with roughly 30 737's would be the sixth-largest U.S. carrier in 20 years. America West didn't even exist, and no one would have imagined that PHX or LAS could have supported airline hubs. The 717, 777, 747-400, Next Generation 737's, A320, A330, and A340 didn't exist; the first MD-80's, 757's, 767's, and 737-300's were just rolling off the production lines, and the L1011 production line was still active.
Consider also that at a relatively modest annual growth rate of 7% compared to historical numbers of 10+%, Southwest will be roughly 4 times its present size and will likely have added service to 30-50 additional cities. jetBlue may well be 10 times its current size in 20 years, and AirTran will likely be several times its current size. Do you anticipate that there will even be an opportunity for an additional low-fare airline to exist in the market by then? Will a start-up low-fare carrier without a large amount of capital be able to compete against the economies of scale which will be enjoyed by the existing large low-fare carriers in 20 years.
15-20 years is just way, way, way too far out on the horizon to do any planning in a consumer cyclical industry like the airlines. You can say today that you're going to want to use 717's, but without knowing what will be available in 2017-2022 and people's travel habits then, it's pretty meaningless.