There is probably some truth to that. A very large portion of Midwest's revenue is tied up in slot controled airports which offers some protection from competitors that want to compete with Midwest. However, that being said, if you believe the press release that AirTran released last week you have to believe that things are different this time around. The last time nobody knew who AirTran was. This time around all the publicity (even if it has mostly been negative in MKE) has been great for AirTran. AirTran is in a win-win position from a publicity standpoint. The hardest part of starting a new market or expanding service is getting your name out there so people know that you offer services. The MKE media and Midwest are doing a fantastic job of letting people know that they have one LCC option in MKE. It is the perfect example of the old saying "there is no such thing as bad publicity" - and it seems to be working very well for AirTran.
Probably the biggest reason that they want to buy Midwest before they go into MKE is so that they would have the means to defend it. AirTran doesn't have the time to build up the critical mass necessary to defend MKE if they enter and are followed in by WN or B6 (which, as I have said before is EXTREMELY likely). If they buy Midwest they would be able to defend MKE against anyone that would come in after them from day one.