"Not surrendering in an unwinnable game can teach you how to defend"
No, it teaches you how much the enemy can kill you instead of ending the game. You can't learn anything in an unwinnable game that you can't from a game where you are just behind, but a good teamfight is what is needed to win. Stop treating unwinnable games as the manual of this game. If a game is unwinnable, give up and move on, you won't learn anything special from wasting time.