First off, let me say this: you can't nail down one particular cause for replay value in a game.
I remember playing games again for cutscenes back before YouTube allowed me to skip to what I wanted, so it's not just gameplay. Hell, I've replayed the Phoenix Wright games enough times, and there's not really too much in terms of "gameplay" there.
In fact, if I'm allowed to be vague, the only requirement a game needs for me to replay it is for it to be good. Something about it has to be worth investing time into it again.
However, I definitely think that DLC has eaten away at a portion of replay value in general.
"Depth" in games is one of those strange concepts that's easy to point out, but hard to define. It's also a bit confusing since it can mean multiple things for games, from how polished the content is to how much of it you have (though said content to have depth must meet a certain standard to put it above "gimmick" or "filler" levels).
It's this vague area that DLC more often than not eats into.
To take a recent example, Capcom recently announced that the costumes for their latest Resident Evil release wouldn't be DLC but rather unlockables. Now, besides for the obvious point this presents that unlockables are now considered exceptions to the rules of additional content, and that DLC is considered the norm, I'd like to point out the values of unlockables to depth.
Firstly, unlockables, by their nature, require input to achieve. They serve as a reward for players, often to those that continue to play a game far past what most would consider its ending. In a way, this adds more to a game and the player's experience without really putting in too much more content.
Secondly, unlockables can be used to change the entire game experience with similarly small amounts of actual changes. Game difficulties impose enough new rules that the experience feels different (yes, I'm hinting at the newest Metro game with this one). Stealth games can change to action games when the rules become more lenient. Action games can change to fast paced puzzles when you have to figure out timing. Hell, a shooter can turn into a survival game (though not strictly speaking an unlockable, look at Arma II and DayZ). These are just difficulty changes. I'm certain most people reading are familiar with the term NewGame+, and can attest that those runs are very different from just hitting New Game.
The final topic I want to talk about is something a bit different. Unlockables allow for creativity unconstrained by the normal limits of the game. Balance, aesthetics, and even lore can be tossed out the window here. Unlockables can be ridiculous or cameos from other franchises. They can allow a game to move beyond what it is and tap into whatever it wants to be.
DLC can replace 2 out of 3 of these things for unlockables, but the problem arises that the first value can be so important to a game.
While DLC secures money instantly, unlockables secure commitment, which comes with more memories about a game. I can almost sort games I've replayed or stuck around long with from those I haven't purely by how much I remember them. These are the games that I'm likely to remain a supporter of, as I enjoy them and want more of. DLC serves as a quick fix while I'm still riding a wave of hype for a game, but unlockables pull me into games and get me wrapped up in them.
I know the industry has moved towards trying to launch products as quick as possible and add more later thanks to the magic of the internet, but the push for DLC has forgotten why extra content exists in the first place (It is certainly not for adding an ENDING later on, Asura's Wrath).
Assassin's Creed's DLC didn't convince me to support them for 4 years. The Tales games didn't munch up triple digits of my life because of anything I could buy for them. My roommate and I didn't destroy the time counter in Kingdom Hearts because of paid content (okay, mine hit a couple of days, but he hit solid 9s on his file).
True loyalty isn't bought (or sold in this case), its earned.