I think you're missing the biggest thing about FCs...it isn't just the fact that they get xp quickly, it's the fact that you pay them and get to level a job while AFK. What does this mean?

Most people think leveling up a job isn't fun, and is a hindrance to the actual game content.

So like I said, the only way to fix this is to make rich low/mid level content that also bestows a great amount of xp on the player. Only then will low/mid-level parties might make a comeback. Even then, human laziness may ultimately win people over and they'll just opt to pay a couple mil so they can cleave up to level 99 in 2 days.