Well, there's no real technical limit on how long a game can go. Nor is there an actual requirement for a person to be logged in for the entire match...they simply do nothing while not logged in. Likewise, the actual protocol is sufficiently lightweight that there can be an utterly ridiculous number of players per match without even worrying about load balancing.
So, you could have a never-ending match, allowing a *very* large number of players...basically like an MMO. Technically, not a real problem, but would it be a problem from a gameplay perspective?
You can only have a character in one match at a time, so any character in the MMO world would be unavailable for regular matches until he left it...and there would need to be a mechanism to leave early if the match never ends. Also, most matches are entirely pvp. A ridiculously large, unending FFA match might be fun in a way, but camping spawns could be a huge problem, and some people might prefer PvE options be available in such a map. How do you all prefer such problems be resolved?
