(2) Even assuming your argument goes through successfully, it is irrelevant to whether the person committed the sunk cost fallacy, which is the issue under discussion. Now, you could argue that subjective assessments of expected value always track the true underlying expected value (i.e. the actual expected value for any individual's objective abilities). In that case, all non-academically-gifted students who chose further education would likely be committing the sunk cost fallacy. That's not the case though, and you know that. Or you should, because it is prima facie absurd.
(3) It is relatively uncontroversial that there is a substantive disconnect between the supply of graduates and the number of jobs paying a salary justifiably commensurate with the debt-levels we are seeing. This has a couple of knock-on effects. For example, an across the board devaluing of degrees, which unjustifiably increases hiring requirements for lucrative professional jobs, further exacerbating prestige whoring, etc. This turns high school seniors deciding to attend elite private schools into even greater gamblers as the cost of an initial degree rises. Maybe they are or maybe they aren't rational to take that gamble (Are gambles by definition irrational? If so, pick a different term.). However, after having taken that gamble and come up short, they don't automatically become irrational in taking it again. Or, at least, the irrationality wouldn't necessarily have anything to do with the sunk cost fallacy.
In fact let's look at it: what's the probability that given your inability to gain one of the lucrative jobs at this level of education you can gain one with a higher level of eduction? Well we can speculate, a higher level of education can be sought for two reasons: you're acadmically gifted and genuinely want to get more education, or you're not academically gifted and you want the magic fairy wand to wave to get into Brothers and Brothers LLP. Well let's face it, you're in group number 2, and it's the people in group 1 who are the actual successful candidates. So given that your pay off looks way off. You won't get the top jobs - you're not good enough, you won't get the medium jobs - you're overqualified, and you won't get the cheap jobs - you can't afford to work for that amount.