That has to count for something more than just a chance play where officiating comes in to play.

Like I said immediately above your post, it also isn't a straight swap. It's magnitudes of order easier to convert a 4th and 15 than it is to convert an on-sides kick if you assume (fairly) that a 3rd and 15 - which is not an uncommon occurrence - will almost always play out the same as a 4th and 15 because after a 3rd and 15 a team will punt 90+% of the time (other than late game situations here the team trails). So 3rd and 15 is a good indicator of how often a team would convert a 4th and 15, and it's much more frequent than teams currently convert on-sides kicks when the receiving team expects it (which it would in any cases where a "take the ball and run a 4th and 15 instead" is in play).

If they want to be gimmicky, they at least need to be gimmicky in a way that doesn't decrease the chance of the receiving team to secure the ball after the other team scores. Like I said in the other post, if they made it say 4th and 30 with any penalties not being automatic first downs but instead just the yardage up to 15 yrd for PI and replay the down, then I could get on board with that.