It's not the monsters' levels that are the problem

Yes, and I even noted it in my post.
But LHC is here to solve a different problem, that of high level people having no place to train at anywhere close to the speeds they could pre-BB.

The "let me train on mobs higher than me" complaint is heard from far lower-level players: level 40, and 70, and 100. They don't lack for places to train, but they feel they could train faster if the formula would only let them challenge higher-level mobs. And the reason they feel that way is because mobs their own level are no challenge whatsoever.

The fastest training is always on high spawn maps with mobs you can one to two hit, up until 4th job when LHC comes in.