Multiplayer on obscure-ish games isn't always abandoned, though. There's still players on 360 TF2, a game that came out in 2007. There's still players on Brutal Legend, a game that came out in 2009 that a lot of people didn't even realize had multiplayer. From what I've seen, the question of whether the multiplayer on a game that isn't CoD will last basically depends on whether that multiplayer is actually unique and fun enough to sustain a player base. Which in those examples, it is. I'd guess that Assassin's Creed: Brotherhood's multiplayer will be the same, as that fits the bill too.