It would be a little more realistic, maybe but... If you think about it, that would mean any airline could have almost unlimited growth. Since it's quite easy to turn a profit with pretty much any aircraft, especially at the start of the game, it would just mean that the MEGA airlines would grow even quicker than they do now and swallow up even more of the best planes. The game would be even more biased to those who could log on and buy three planes every game-week, day and night. (I kinda include myself in that group, and I know it's what I would do to get a competitive edge).
The current way of obtaining aircraft (which has evolved many times over the years) does at least slow things down somewhat.