So, first off - no thanks. Onside kicks are supposed to be ridiculously hard. That's the point. It's to minimize the chance of "gaming" the system (just like bball needs to find a way to end the ridiculously called fouls at the end of games by the losing team, but that is a different rant).
Onside Kicks have a success rate of roughly 7%.
4th and 15 success rate was closer to 25% (23% per the article linked at the bottom).
I realize it's only used once, and in the 4th quarter - but I'm not for giving a team a near 25% chance of getting the ball back.
Maybe if it were closer to the 15% success range, I'd be OK with it. Maybe that would be the equivalent to 4th and 20? 4th and 25? Not sure, but I'm not a fan of this proposal as it currently stands.
Proposed "4th and 15" onside kick alternative can only be used once in the 4th quarter