I've seen this argument pop up countless times on this thread.
1.Once more from the top, giving the worse player a frame-9 instakill move that true combos out of both a frame 1 jab and frame 3 D-tilt as a reward for being worse than his opponent is not and will never be a proper balancing factor/buff. Comeback mechanics are just that, an ability for the worse player to come back win. This has happened both in and against my favor more times than I can count, and I'm very surprised that anyone else is defending it.
2. In tournament, I have consistently survived to over 80% almost every stock, and whenever I don't it's due entirely to my overextension which is a fault of my own, not the character. Proper combo DI and jump conservation can get him back from a much larger amount of situations than he lets on, I even surprise myself sometimes when attempting to recover. Pair that with Flaming Straight Lunge, and he gets a much better ability to recover. Basically, not playing smart is what gets him gimped. And if you're not playing smart and getting gimped, then I see absolutely no reason why you should be getting an instakill move over someone who's actually outplaying his opponent.
3."A good player won't get hit by it." This is only true if the Little Mac player isn't also a good player and can't properly read his opponent's fear reactions and habits. Any player worth their salt should know what their opponent will do in reaction to fear, and assuming the LM player was good enough to convincingly take the opponent's stock and get the KO punch with a stock lead, then he's likely going to be able to make a simple spotdodge/roll read to hit a few d-tilts. Even good players have habits,a nd if you're able to read those habits, then you should be the one getting the instakill move. On top of this, considering how safe, fast, and low-commitment all of the moves that combo into it are, I'd say it's pretty easy to get a lucky D-tilt into KO punch against a player who's better than you. Believe me, I've seen it happen. You see some crazy stuff in rotations.
One final thing: people seem to not notice that if it were reversed, you would still be able to make comebacks with it. LEt's say you get Mac edgeguarded on your first stock, but are able to keep your composure. After that, you take their first stock with yourself at 70%. A bad position, but you'd still have the KO punch, land it, and make the 2-stock comeback even after getting edgeguarded.
Additionally, after thinking about it more, having the numbers reversed would actually benefit Mac much more against campers than evening them. THinking back to a set I had against a campy Luigi, my main gameplan was to consistently D-tilt his fireballs, stuff his grab approaches with D-tilt or jab, then eat through his aerial apporaches with Up-smash to build KO meter from the damage taken. Then, I would wait for him to be at a good percentage to go for a hard read to kill, or wait until I got the KO punch from the repeating process. However, I would have gotten the KO punch at least twice as fast if it was reversed, as I was getting ~7% damage dealt for every fireball he threw(Which was upwards of six for every time we were in neutral), 15% from every time he apprached with grab(Even more if it was a D-tilt due to conversions), and upwards of 30% every time he approached with an aerial. I guarantee that I would have had at least six KO punches that set had the meter been the way I'm proposing. Same goes for Rosalina;I hit both her and the luma? That 30% D-tilt to D-pivoted F-smash becomes ~50% for the meter, as opposed to taking more damage that puts me in a bad position and sets me up for a gimp. Everyone's talking about how he would get gimped before doing enough damage to fill it, though they're ignoring that making it so that you have to take damage to fill it litterally guarantees it to much more of an extent. Either you maybe take damage while going in on them and taking a stock in a 0-death, or you are forced to take damage to get the same reward, which, according to everyone's statements, is much more likely to set you up for a gimp than the latter.
TL:DR:Reversing the numbers of Little Mac's KO punch meter would be a straight upgrade that would benefit him far more in bracket than keeping them the way they are. I know this as complete fact due to extensive testing in tournament and in the lab, as well as the agreement of other proficient Macs such as Doom.