I'm only level 12 right now on TLoU MP. The online is pretty good, but the fact that there's really only one mode limit replay value to me (I realize there are two, but they both sport the same objective).
I know you didn't want the "fun" answer, but that's what it is.
I played 800 hours of UC2 online. Now, somewhere along the line, I pretty much saw everything Iw as gonna see. But it was still FUN to play, so I did.
With TLoU, I've recognised there are only two modes, like pdiss mentions. So, I play for an hour or so, then I simply turn it off. This will ensure I don't burn out before more modes. And we will have more modes.
I don't care for Co Op infected modes. Not sure I want to do that. But I want some Objective modes of one sort or another.
Here is the thing, Playing TLOU's multiplayer today seemed like a chore after getting the platinum last night. Don't get me wrong it's still better than most MPs I've played, it's just the lack of.. incentive to keep playing. What keeps you playing? Aside from the cheesy "fun" answer.
Why do you play any game? If you are doing it for the trophies then that would seem like a "chore" to me. Instead of enjoying the game you are tasking (Chore) yourself with doing certain things inside the game taking away from getting lost in the game and having fun. Sure, being a completionist can be fun, but if your only goal is to complete something, than of course once you "Complete" the objectives, you will feel there is nothing left to do.
So while you may think "Fun" is a cheesy answer, it is THE answer to the reason anyone would play any game (or replay a game). If there is no fun, whats the point in playing.