He doesn't lose his last year salary, he loses his first.
If he stays he will lose his 2022 salary, 200K or whatever number that is, not his 2032 salary. His body will last for 10 years, let's say.
I understand you are saying you can develop better on a foreign pro team and you will reach that full potential one year earlier.
You might be correct, but if we assume the coaching, facilities, strength and conditioning and nutrition are the same here as in a mid-level Euro league and he improves a lot over the next year and starts at 300K rather than 200K then he could reach his full $1 milllon potential at the same exact time in his 10th and last year. He loses 200K on the front end of that not on the back end.
It all comes down to where you can develop better. I find a hard time believing with coaches making significantly less money then our college coaches and without the strength and conditioning and nutrition opportunities available here that it is clear cut. I think it depends on each unique situation. As a grad student here, he could easily have more free time to devote to strength and conditioning then an undergrad would if he wasn't actively pursuing that degree.