Yes, the statistics regarding the conversion rate for third-and-15 and fourth-and-15 roughly suggest that an onside kick and its scrimmage-play alternative will have the same success rate. But there are two reasons to believe that the success rate of the fourth-and-15 play as an onside-kick alternative will be even higher.
First, teams usually don’t try to convert third and 15, as explained by 49ers coach Kyle Shanahan in December, after the 49ers converted a pair of third-and-16 attempts in a Saturday night win over the Rams.
“[U]sually I get in I- formation and run the ball out of frustration, but that would have been bad in that situation so I’m glad I didn’t,” Shanahan said at the time.
So while on third and 15 the offense often is trying to gain back some yardage in order to enhance field position for the ensuing punt, a team that goes for it on fourth and 15 is trying to convert.
Second, if the fourth-and-15 alternative to the onside kick is adopted, teams will have reason to devise a play aimed at gaining the 15 yards. Currently, not much time is spent on an all-or-nothing attempt to gain that much real estate in only one play. If/when the fourth-and-15 play becomes a thing, teams will spend practice time on the play.
Which will, as a practical matter, make teams even more prepared for a fourth-and-15 play that may arise in the normal course of a game.
Defenses obviously will spend time preparing for a fourth-and-15 play, too. And that may balance things out a bit. Still, given that the offense knows what it’s doing and the defense is constantly reacting, having both sides of the ball ready for one snap aimed at gaining 15 or more yards will likely benefit the offense more than it benefits the defense.