This is just the price of freedom at this point. I recognize that's the line gun advocates use when someone shoots up a school, but you really can't put strict restrictions on rental vans. They're $20 a day to get.
I agree... you can't protect people from every conceivable threat, and there's a point at which trying to protect people becomes a bigger threat than what you're trying to protect them from. I think trying to restrict vehicle rentals on the minuscule chance that somebody might be planning a vehicle attack on pedestrians is an example of such a situation.
Focusing on identifying potentially dangerous individuals makes more sense to me than trying to eliminate every possible means of attack.
Unfortunately, in this case it probably wouldn't have helped. In the case of this individual, interviews with people who've known him had no indication that he had any interest in violence at all. It sounds like he may have had Asperger's, but that's certainly no red flag for violence.
-k