If you receive benefits from society, no matter what they are, you make more money. So why not just assume that the more you make, the more you've benefited from what society offers, whether education or anything else? Then you just tax people who make more money, and tax the higher the more they make. Which is just regular old graduated marginal rate income tax.

Instead of the false assumption that the more you make, the more you've benefited from what society offers, whether education or anything else, we should make the true assumptions that

a) a subsidised university education is a significant, measurable benefit.
b) receiving a university education correlates positively with being well off, and so if anything the raw cost is more likely to be an underestimate than an overestimate of the correlation between having gone to university and amount of benefit.

Even if you start a company with a high school diploma and make billions, likely thousands of college graduates help you out along the way. A progressive tax system is vastly more popular and fair than an archaic graduate tax system. The system would seem to be vastly regressive as many degrees don't earn nearly the same as others although they do benefit society...

Let's go for a 60% young earth creationist nation! 40% is way too snooty ivory tower book-larnin' elitist for us!

You are taxed if and only if you graduate from a university? What if you get a degree in anthropology or medieval left-handed monks? Any of the many very fascinating majors that don't lead to good jobs? What if you never graduate from University but wind up making billions of dollars? Bill Gates comes to mind here.

Public universities and colleges should be free or very low cost for the first six years you attend. If you take longer than that to get a degree, you pay. Graduate school should also be not very expensive, and with a realistic time limit imposed.

Meanwhile, a graduated income tax is always a good idea.

And in most circumstances, it's much worse than any actual tax out there.

There is one practical difference, and one psychological.

The practical difference is that the amount you pay scales radically with your future earnings. In principal, debt repayments could do that to; in practice, it's much harder to sell.

The psychological difference is that even though a debt you'll only have to pay off if you get rich may be functionally equivalent to a higher level of income tax, it seems to be much more of a disincentive to apply to university.

I'm for free lifetime education for everyone, paid education for anyone in fields where there are shortages (primary care medicine, teachers in rough urban neighborhoods, family planning worldwide...) and a steeply progressive income tax that makes it impossible for anyone to become so wealthy they can corrupt the democratic political process, or for anyone to be left behind illiterate, without good food, safe shelter, and appropriate medical care.

You make more money, you pay more taxes. Lets not complicate the tax code any more than it already is.

Too complicated currently.

We already have a progressive tax system. Just make it progressive with a few more tiers and that's it. This idea nearly has the effect of stigmatizing the educated.