I think comebacks are very possible. Respawn timers past 25 mins get pretty big. Before that, probably not large enough. My team had won every team fight before that, but by time fight is over, and you push the lane, early on somebody has respawned and can defend the tower, against half dead people. After 25 mins though, 1 teammate got caught out, we should have let him die, but we tried to go in, and ended up all 3 dying. They were able to push down 2 towers top, and inhib, before we could even respawn. Early on, I feel like you can't punish the enemy team after killing them all. All it takes is that one late game mistake, and you lose everything.