A lot of smart people get degrees to make money. I get that people with a college degree, on average, make what now will be something like a million dollars more during their careers than non-degree earners.
ETN's is in a different situation. He will likely be a 2nd day choice - about as good as it gets for a top but not generational RB talent - this year or next. By waiting, however, he loses this year of salary (around $500,000) and then (assuming he has a very good pro career under either path), in 2024 he will get, by staying in school for 2020, this $500,000 as th last year of his initial contract instead of the maybe $4 million or whatever he gets per year for his second (and more important) contract. He is also a year older when signing his second contract, which for a typically shorter career RB, will likely decrease the annual value of that most significant career contract over its term. That's the likely earnings loss during his career if he remains a top talent (roughly $4-5 million at current salaries). His post-football career is going to be determined largely by his pro career and isn't likely to be changed much by a college degree (which he can pursue while playing football if he really wants it).
If he now has a terrible senior year or gets badly hurt, the amount of his initial contract will drop dramatically, which has been shown conclusively to drop (on average) his career earnings in the years beyond his first contract. That would be a huge loss compared to earning $500,000 in each of his first 4 years under an initial contract. If get gets so badly hurt that his career is over, he will be that same $500,000 ahead if he'd signed in 2020 and it happens during his first 4 years, and the same $500,000 in 2020 and same $3.5 million or so ahead if it happens during the second contract. There is virtually no scenario where he isn't hugely ahead financially by foregoing his senior season to sign now, while on top of the world.
He could, of course, become an even better RB than top 2 or 3 in the whole country over the next year. If you follow this stuff, that doesn't happen very often. If he is not a projected as a generational talent at 20, he is not at all likely to become one at 21. The odds of decreasing his draft status by waiting, for someone who is already at the peak of national talent, is far more likely than the odds of increasing his draft status. Most likely, however, is that he is as good as he has appeared to be, and will have a similar projected talent going forward - his projected career value just being dramatically less, for having started earning a year later and having a year fewer of top earning years.
And he can, of course, make he decision to go back for his senior year at the anticipated cost. I'm sure he loves his team and that the desire to win a championship calls to any player who falls short on a top quality team. The odds of any one team winning it all next year, however, are long and the cost of that commitment (in his case) huge. I think most of us just fear that everyone around a kid in this situation wants him back because that increases the schools's situation and coaches careers, and we wonder if the kid is getting the kind of neutral guidance he should on the effect staying is likely to have on his career earnings. We also think he is good enough to play at the next level and would like to see him take that challenge - but that's out issue, not his. Its completely his choice, of course, we just hope he is getting good advice about the consequences.