You can't assume that the teams are going to be perfectly balanced because of the existence of elo. In one of my games the other team had a Janna who was diamond in 2014 (had the avy). Also, when your Malphite gets camped because he refuses to ward and constantly pushes up to turret, then blames you for not diving his full health Nocturne as level 5 Jax, there isn't much you can do when he proceeds to go 0/10. You could be fucking Faker and if your team just refuses to group, you're gonna be trounced in the ensuing 3v5.
Let me fucking tilt in peace.
ive won 2v5's when smurfing at gold elo(literal 3 dc's), as long as you have a hypercarry champ( like gangplank..), you can win even in the most ridiculous situations if youre significantly more skilled than the opponents.