I’m a big fan of this rule. While winning the game in the final minute shouldn’t be likely, it also should be impossible. Id also argue there’s far more luck than skill involved in the current onside kick format. Changing it to 4th and 15 shifts the needle closer to skill.
But the reason of the rule would be to make it less impossible than an onside kick, not more. 4th and 25 and you might as well keep the current format. They didn’t arbitrarily pull the 15 yards out of thin air. It has very similar conversion percentages of the old onside kick I believe.