Growth is so rapid mainly because at the beginning of the game there is no incumbent competition. If tomorrow united, delta, american, us airways, southwest, jetblue, airtran, and alaska all went out of business, I bet with in two years there'd be at least one totally new airline in the US with at least 150 aircraft.
There are negative consequences pretty much immediately if you go into -cash on hand. You could lose aircraft being used to secure loans, you can't open new routes, and after a certain point you can't do heavy maintenance. Eventually they shut your airline down. Hope that answered some of your questions.