Your points aren't wrong. But one thing you aren't factoring in is that if he comes back, he takes 0 dollars next year and his rookie salary clock starts one year later. If he goes pro and does incredibly well, such that he can sign a big contract after his rookie contract is up, plus the inflation going on...I think the bet on yourself is actually take money now and get to your second contract ASAP. Basically, if he can make the difference of 28M and 10M (that is, if he can make 18M) in year 5, he is actually ahead leaving this year. But I see your point.
You're correct! He may be ahead in getting to that second contract BUT in the end he will have made more money when it's all said and done if he stayed and got back to his original projection. That extra $14M won't ever be made up. Let's say he plays 20 years in the league (if he leaves early) but 19 if he leaves late. The chances of him making $14M as a 20 year vet is slim. So he still comes out on the plus side.