Military comes from higher-income backgrounds because the opportunities out in the economy do not support the continuance of that lifestyle?
Not saying all, for a small part of the enlistees it could be a factor.
The broad brush approach to the reasons for enlisting is wrong no matter what reason is chosen.
That might be true if your bleak (and incorrect) view of the economy were accurate. But the more important factor that refutes your theory is that every one of the young men and women volunteering in the past five years knows that they are most likely going to have to go to war for their country.
It is highly unlikely that any intelligent, educated, moderately wealthy young man or woman would give up their comfortable lifestyle (which they know they are doing) for any reason other than a desire to serve and a love of their country. They know that we are at war against terrorists, and they want to help fight them.
And they are also reinlisting in droves because they want to continue to serve, and they want to go BACK to Iraq.
So not only is your argument wrong based on the economic reality of the Bush Boom, but it is wrong based on the self sacrificing requirements of being in the military that all these recruits now know about.
It's not a matter of broad brushing anything. It's a matter of being factually accurate, and your hypothesis, based on a Viet Nam era draft mentality, is not.