They were really extremely straight forward, and linear as well. It just wasn't as obvious.
I find them more replayable as their battle and ability systems were more readily open earlier in the game.
It took far too long in XIII for the crystarium to expand, and battles to become fun with more abilities to mess around with.
That's a very fair point, and I guess I don't disagree. If it's for battle purposes no one can say it has no replayability.
If the gameplay is very fun, anyone would want to replay it. If it's only the story that compels you, well I don't think old FF's are that different from FFXIII outside of battle. So that's why I don't replay them. I'm mostly invested into the story since FF8, yes I started after FF7 and never picked it up. I think it's too late for me to enjoy it.
And ever since then I didn't feel a great sense of Depth, so I only play for the story. but understand once you play a game as deep as say Super Smash Brothers Melee at the highest level, or any competitive game like Starcraft at a high level, FF games even the old ones are cake walk. This is just my opinion obviously.