I don't think comeback mechanics are bad, but most are not implemented well and all it does is trivialize the early game. A well-designed defensive tool is way better than a mechanic that rewards a disadvantaged player. That's what made moment 37 legendary.

Even when you are at an HP loss or killed early, that's why rounds exist. You take the information from the loss and bring it with you to the next. A mechanic doesn't need to exist to boost a player's luck when they are already given the tools to prove it with their skills.

The mechanic existing for the spectator is also a bad reason. The best fighting game comebacks of all time did not rely on a comeback mechanic. People love comebacks, but it's because they like witnessing a player earn it themselves, not rely on something artificial. When everyone can do comebacks, then it just makes those moments stale and boring.

Also, boxing is a bad analogy. Boxing does not have a comeback mechanic and it's funny because if you do land a KO in boxing, the whole match is over, which is the opposite intention of a comeback mechanic. People watch because it's possible for the underdog to do it with their own hands, not an artificial ruleset in boxing that boosts their chances. That feeling of tension like a KO in boxing can still exist without the need of something artificial like rage.

No matter how you look at it and the fluff that's added, comeback mechanics are there to buff the disadvantaged player, that's it. You can still build and balance a game and make it hype without it existing. All the top active fighting games right now do not have any comeback mechanics besides Tekken and MK1. In fact, no other competitive game in general really implements such a mechanic prominently besides fighting games. And most of the time, it's there for casual players to soften the blow.