You're mixing income tax with consumption tax. If you want to tax income, tax income. But if we're taxing consumption, then those who consume more pay more than those who consume less, and those who consume least because they have the least to spend pay the least in taxes because of the baseline exemption. The upper middle class guy who's busting his butt to pay off his student loans, and who saves as much as he can, is not going to pay a disproportionate amount in taxes.
The only reason the particular middle-class guy in your example isn't paying a disproportionate amount in taxes is because he took out a bunch of loans before he became middle class, and thus shifted his spending backward in time to qualify for the exemption.
Or, another way of looking at it would be to say that the guy with student loans isn't really middle-class yet, because $0 income for 4 years plus $75K income for one year averages out to be "equivalent" (in some sense) to a guy making $15K each year. And if the guy had $60K in loans to pay back, and they had to be paid back in one year, then it pretty much would be equivalent.
But I digress: even in the case of somebody with student loans, there are only two possibilities: either the loan payments are a large fraction of income such that the person is "poor" after paying them as in the example above, or the loan payments are a small fraction of income such that the person is still "middle class" after paying them and is inflating his lifestyle and thus paying a disproportionately tax rate anyway.
If he pays off his student loans and still chooses to live modestly, he continues to pay a low tax rate. If he instead decides to start living large, then he'll start paying more in taxes. Either way, he essentially chooses his tax bracket, because he chooses every day what to buy, and how much to spend on it.
Here's the problem: on average, nobody does that! Did you see the graph I linked? The average savings rate, across the entire bottom 90% of the population, is about 2.5%. To say "just save more" is a non-solution because it ignores human nature.
Now, you're right that some people -- weird people -- would save a large fraction of their income and thus make out like bandits. As one of those weird people myself (I'm a big fan of mrmoneymustache.com and plan to be wealthy enough to retire at age 45 or so), I completely agree that it's entirely possible to do. However, I also realize that, when considering society as a whole, people like me are a negligibly tiny fraction. What's advantageous for me and what works for society are very, very different things.
This tax is progressive in that people who choose to live modestly or who cannot afford to live extravagantly pay very little in taxes. Those who are able and choose to live extravagantly pay much more in taxes.
You're ignoring the third category, which is "people who can afford to live comfortably" (but not extravagantly). (And by "afford" I mean "live paycheck to paycheck, saving very little" -- which is a poor definition of "afford," but nevertheless the one most people use.) That category describes pretty much the entire middle class (again, except weird people). Unless you make the exemption cutoff so high that only true "luxury" goods are taxed, they are screwed by your plan.
Finally, although up to this point we've been avoiding discussing why it's important for taxes to be progressive in the first place, I'd like to reiterate that is is indeed important. Money is power, and without a way to siphon off and redistribute excessive wealth, runaway compound interest allows the most wealthy people to become so obscenely powerful that it destroys the political system (and eventually society itself). It happened before the French Revolution, it happened before the Soviet Revolution, and it's happening in the US right now. Insufficiently progressive taxes are dangerous.