To be clear, I'm not even arguing that the Giants should have drafted a QB. I'm just arguing that they should not have drafted a RB. Saquon was not a sure thing, and even if he turns into a superstar, you're still paying him on his rookie contract what he stands to make as a superstar, so there's no real added value.
Let's say for example that Saquon had a 50% chance of being a superstar RB, a 40% chance of being an above average starting RB, and a 10% chance of being worse than that. In this case, you have a 50% chance of overpaying him on his rookie contract and a 50% chance of paying him what he's actually valued at.
Now let's take a QB. Let's say the QB has a 10% chance of being a superstar, a 20% chance of being an above average starter, a 40% chance of being a below average starter, 10% chance of being an adequate backup, and a 10% chance of being out of the league entirely. Because of the way rookie contracts are structured, you have a 10% chance of overpaying the QB, a 10% chance of paying him his actual value, and an 80% chance of getting a bargain.
The same is true for basically every position besides RBs; not to the same extent that it is for QBs, but literally any position besides RB, K, and P makes sense in terms of value.
Point being, if the Giants didn't like any of the QBs, that's fine. There's still no scenario where it makes sense to draft a RB at 2 anymore. Finding good players on cheap contracts is probably the easiest way to win in the NFL and with the rookie salary system, the NFL draft is the easiest way to find good players on cheap contracts and the Giants just forfeited a golden opportunity for that.