You're right.
I never understood - especially back in that day of "not nearly so big contracts" - Humphrey jumped so quick to the NFL.
I understand why some think Hump should have stayed another year to improve his draft status, but the thing is, it's a huge risk. Let's say he gets better and is top 10 material. Then he goes from his $12M contract to a $20M contract. That's great, right!
First, it's a big assumption that he'd improve above being the 16th player taken.
Then other side of that is guys like foster, Jon Allen, and more specifically Eddie Jackson.
Fosters shoulder injury and drop down in the draft likely cost him $10M, with the same probably true for Jon Allen since both were likely top 10 picks without those injuries (assuming those were not there before their senior years, which might not be true.)
Jackson got a contract for $3M, after being projected as a likely first round pick after his junior year and going into his senior season. If he's just a mid-first rounder, it cost him $8-9M, toward the top of the first round and it cost him close to $20M.
So there's risk involved in staying, big risk. The real deal is that the difference in your life for a $12m contract to a $20m likely isn't that high. Both of those contracts are in the range of "never have to work after football" if a guy is smart with his money. $3m is not in that range. A $3M contract and a guy has to hope he makes it for the next contract and/or have a really good plan for what his post football career will be.