It's not another possession, it's another play with a shot at keeping possession if they convert. So just because they are losing they shouldn't get the chance to gain 15 yards on another play? Under that logic it makes no sense to even let them attempt an onside kick now because technically that gives them another chance at recovery and maintaining possession. Maybe the winning team should stop them on defense. It adds more excitement to the game. I don't know about you, but onside kicks are boring as hell to me unless it's a surprise onside and with the low recovery rate and all this preaching about player safety, eliminating it is the better option