The 4th-and-15 alternative to the onside kick, as currently formulated, would allow a team to use it up to twice per game, even if the team is leading. And that would be a mistake.
The scrimmage play is being proposed primarily as a way to give the team that is trailing a chance to close the gap, a better chance than the current onside-kick configuration that, given changes to the formation, has made it much harder for the kicking team to recover. It therefore should be confined to the team that is losing the game and, in turn, trying to tie things up or take the lead.
The fact that the clock will run coupled with the ability to use the 4th-and-15 play while leading would give a team an opportunity to easily cement a win after going ahead late. In lieu of kicking off and giving the losing team a chance for a Stanford band-style play, the team that takes a late lead can opt for the 4th-and-15 play, burning off the last few seconds with the quarterback running around and/or throwing the ball deep, like Patrick Mahomes did to ice Super Bowl LIV.
Speaking of Mahomes, the fact that the Chiefs already are drooling over the 4th-and-15 possibility for retaining possession could present a different problem. Football could essentially become “make it, take it” for the Chiefs, and a 7-0 lead could quickly mushroom to 21-0.
The fix is easy. Either prevent the team that is leading from using the 4th-and-15 play or make it an untimed down. Any other approach defeats the overriding purpose of the play: To give the team that’s losing a chance to win.