All that makes sense about the probability, until you have a teammate have it happen to them, i.e. Eddie Jackson, and you're probably hearing from some folks you're good enough to get drafted higher (which he was, since he ended up starting), and not really listening to the advice of others saying you might not be that high of a pick this year.
That said, the lesson is still there that the players should probably choose who to listen to a bit more carefully. There's a reason Saban keeps the program so open to scouts and it's to get their honest feedback about all the players and where they stand.
With Smith, he was going to lose his starting job anyway so he had nothing to gain by staying. He'd already lost the job to a true freshman and the only reason he got it back was due to Diggs going down. With Diggs back and Jobe coming on strong, and Surtain on the other side Smith was, at best, the 4th corner and was not looking at much playing time.
Regarding the decision-making process, what you're really talking about is downside risk. Making the decision on pure probability assumes an infinite number of trials, or at least a statistically significant number of them.
That assumption doesn't hold in the decision to go to the NFL. There, you have one and only one shot. And if you pass it up at the end of your junior year, then blow out your knee during your senior year, you go from a 3rd rounder making several million dollars a year to a UDFA just hoping to make the team. You can, however, mitigate that risk with an insurance policy.
To illustrate with a ridiculous extreme: Suppose you have a choice: You have a one-time chance to draw a card. You have a 75% chance of winning $1 Billion, but a 25% chance of nothing. Or take $50 million and walk away. The draw has an expected value of $750 million (75% of $1 Billion). So pure probabilities would heavily favor taking the draw.
But it's only one chance, the downside risk is huge, and $50 million is still generational money. So I'd probably take the 100% chance of $50 million, rather than risk a 25% chance at being a working stiff for the rest of my life. But that's just me...others might see it differently.
So I understand both Harrison's position and Saban's.
As others have alluded, I wonder if there's something personal behind this. Saban essentially publicly naming Harrison, and Harrison firing back the way he did. Neither looks good here, and one explanation would be that emotions are involved.