The more I look at it, the more I really fall on Williams' side on this one.
I agree with his statement that he wasn't likely to significantly improve his draft stock by coming back. Maybe he becomes a little better blocker, but he's not going to suddenly become a feature back and rush for 1,500 yards. Playing his senior year maybe gets him into the 6th round. That could be impacted next season by how many NFL vacancies are created and how many of next year's players come out, but it seems more likely he'd fall than rise. And, the likelihood of making a team isn't that much better for a 6th/7th round pick than it is for a UDFA.
So, the biggest benefit of staying is probably to finish his degree...which he can do without staying with the team. I'm not sure how close to finishing he is, but since he's been on campus for 4 years, I'd assume he's close. According to the media guide, his major is humanities. It doesn't indicate his concentration area, but I think it's safe to say that even with his degree he's not going to make a pile of money in his first job. Think teaching salary, not engineering salary - $40K, not 80.
His fiancee (wife?) already has a job and a salary, and I understand he's been living with her while he's in school and playing. Probably living pretty frugally with two adults and a child on one teaching salary. A second child would probably force some tough budget decisions. Except...he has this opportunity to make some significant extra cash. Should he stick around and play for another season - with the incumbent risk of injury, and no significant increase in the potential reward - cutting corners and just barely getting by for a year? Or, should he gamble right now?
If the gamble doesn't pay off...they still have her salary, and are not really any worse off. If it pays off a little, he makes $40K in 6-7 weeks, his career ends, and their condition is - at least temporarily - slightly improved. If it pays off a lot, he makes $400K on a rookie contract in the next year, and they're in a much better position.
For his situation, it seems like he probably made the right choice. There's no significant benefit to staying. Leaving carries some risk, but if it doesn't work out, his situation isn't really worse than it already would be. The potential upside is big, even if it's a longshot.
His degree situation is somewhat of a factor, but it's pretty likely that he's close to finishing, and it's not very likely that his degree choice leads to a high paying job. So again...the potential upside of leaving seems to outweigh the benefit of staying. Even more so if he's less than a semester (or summer) from finishing his degree.