I think your observation about Five Guys' fries applies to too many fast food outlets and is spot on. Ever since some idiot with too much money and not enough brains started a nationwide ad campaign in the late 1980s not-so-subtly titled "The Poisoning of America" that made a villain out of using animal fats in fryers, fast food French fries have been terrible, IMO. Limp and flavorless.
Animal fat added flavor and crispness to French fries.
The ironic thing is, restaurants switched to using hydrogenated vegetable oil, which contains trans fats, and trans fats are even worse for the body than saturated fats. But too many restaurants are still scared of adding some animal fat to their fryer oil.