I feel like the leveling argument still holds true for the earlier games, maybe not FFII so much, but FFIII and IV specifically since leveling is a mind-numbing task in those games once you reach what the developers felt was an "optimal" level. Not to mention in FFIV's case, levels past 70 randomize your stat gains and can actually lower your characters stats, meaning a Lv. 99 Cecil can potentially be weaker than his Level 70 version. In FFIII's case, your Job Class is often just as important as your level. A classic team of Warrior, Black and White mage with either a thief or Monk, even at maxed levels and the best gear they could have, would probably lose to Cloud of Darkness fairly quickly.

With FFV and beyond, the games became more about the customization systems so levels are irrelevant and starting with FFVII, getting to level 99/100 is hardly the chore it was to max out characters in the first four games, but it also lessens their importance. Leveling to conquer a villain works better in series like Dragon Quest than FF, since gameplay mechanics are what usually lead to quick victories. Fighting Kefka and Sephiroth with a maxed level party but without magic/materia/endgame gear can still be a tough fight for the player whereas a level 40-50 team with all the broken spells, abilities, and gear can destroy them in one round. It's why the Orphan is tough argument is kind of hollow because it's rather well known that Poison makes him your bitch and at that point you can basically crush him with a team that doesn't even bother with half the Paradigm roles. He's a bitch without the poison strategy, but you can make the same argument about most of the FF bosses, once you drop the broken spells and abilities, suddenly they become a bit more menacing, even with high levels.