You don't have to have a tank to win. The problem is people don't know how to play with the comps they pick.
Yup. I love when people choose teams with no cc and no tank and want to gropu as 5 vs amumu leo mf. Like wut?
It's incredibly dangerous at Diamond and above to not have a tank. Basically you need the game to end within 30 minutes or you'll get outscaled hard. I've lost quite a few games because of no tank. -_-
Enjoy your leaver
When you're the ONLY tank on your team, and the enemy team is four tanks:
https://s-media-cache-ak0.pinimg.com/736x/d7/92/ac/d792acbf52b5e5618c1ac4cd615e386c.jpg
I've won plenty of games without a tank. But those were games that ended really quickly, usually ending before 25 minutes due to pushing huge leads early on.
But if a game reaches the 25-30 minute mark and you don't have a tank, well your screwed.