Even if the Saints go with a rookie. They will eventually have to pay him if he works out. A QB is a lot like buying a house. You complain about how expensive it is, but the longer you wait the more expensive it is. Underhill pointed out that if the Saints gave Carr $38 million, after the Burrow and Hurts contracts, that would put him in the middle of the pack, which is about right. QBs aren't cheap.
If the odds were better that a drafted QB would work out better, that's one thing. But, they're not.