I see your point, though I'm not sure I agree. Even replaying chapters on Normal sees us getting the same augmented EXP and AP, and doing some stuff (e.g. getting the other dresses or seeing the other Chapter 14 conversation variants) doesn't fall so much under "bonus content" as standard content you could have gotten on your first playthrough.
You don't even ever have to play specifically Hard Mode to get these things, but if you do get them, you will be receiving cranked-up EXP and AP that gets you closer to what I would argue is the developers' intended playstyle.
While we're on the subject, though, I think it's also worth consideration that games -- and especially FF games -- generally aren't known for locking their biggest optional content behind what essentially amounts to a New Game+ feature. Here, though, you can't even fight a Malboro, master the Enemy Skill materia, complete your Assess list, etc. without going into Hard Mode.
I mean, if you're speaking to just finishing the game on Normal, then whatever, yeah, you can blunder your way through a lot of it and leave a lot of content forever unaccessed. Doing an actual complete playthrough, though, is going to require making use of some intended playstyles the developers definitely shove you towards.