In my opionion, I can see obvious advantages and major disavatages to all this merging. Maybe I just always try to see the good points who knows. Some good points of these HUGE airlines are new routes to cities or counties that are not currently being served. Improvements on FF miles for passengers and easy connection of flights, baggage and improved schedules. I think the smaller airlines can survive just fine as well if they can offer a good product in which the customers and employees can form a bond with them. In a smaller airline this is a lot easier then a mega carrier. Some airlines like Piedmont, the new Midway, Jet Blue and Southwest have been sucessful in this style of business.
We will begin to see frustrated passengers at major hub airports around the coutry because of an increase of traffic delays at the key locations. Charlotte would be one of these airports if United wants to expand drastically. With increases in ticket prices, lost baggage, poor service, over crowded hubs people will begin to choose the smaller carrier or drive to a less crowded airport with more competitive prices. For example this is already going on at Raleigh Durham over Charlotte. This will only help out the smaller airports in my opinion. Finally, these maga carriers will begin show signs that they have grown to large to fast and will begin to streamline its fleet and over all size down. Eventully, the industry will bring us with about the same ammount of airlines as we have today but with different names. Airlines such as Southwest, Midway, Air Tran, Jet Blue, Alaska, Frontier, may be playing the parts of todays, US Airways, Continental, Northwest in the not too distant future. Wouldn't it be kinda ironic for these large airlines getting swallowed up by names like Jet Blue and Frontier? Its sorta like The big fish eating a baby fish's older brother. Then the fish grows up and look who's invited over for dinner.
Who knows, its a tricky industry. What logically makes perfect sense never seems to happen when we all sit back and look at things. Its fun to try and figure out though.