This is the most bronze thinking I've ever heard.
First off, if you want to insult someone, look in the mirror and start there - you still think you can't win a 4v5 scenario.
One ADC getting caught out is NEVER a game thrown. You still have your APC and your jungle to do more than enough damage to be able to hold a team fight under turret, especially if you "completely have the upper hand."
Now, maybe you're new to this - and that's okay. But what you have to realize is that person who got caught out made a mistake. Guess who else makes mistakes? Challengers. Professionals. They make mistakes and their job relies on them winning. You're here, playing a free game in what I presume is bronze tier of play based on your attitude that "oh, one guy died, better lose. Can't win this."
Let me go ahead and link you this 1v5 penta kill clean up by K4tarina, an S4 mid-gold Kat main on NA and EUNE. This happened after a 0 for 4 team fight and pushing into base.
https://www.youtube.com/watch?v=RP8w2ks3gGE&feature=youtu.be&t=25s
Why am I linking a seemingly irrelevent video? For starters, I see one massive mistake here: the entire enemy team base rushed when 3/5 of their members were under half health and Kat was up and the two that weren't half health are not surprisingly squishy. Why is this a mistake? Katarina relies on cleanups for resets. Given that her ult applies grevious wounds < 50% health, easy kill on mundo and Leona goes down in one quick burst. Giving her 3 easy kills on the only members of the team with useful CC on Kat paved the way for an easy penta finished on the two carries with the resets.
I'd like to bring particular attention to the game timer. 30 minutes in, team pushing for the win. That's how you win games. You have a clear advantage, you close them out.
Did this play with them that game? No, they still lost it. Was it good enough to make the big plays spotlight? Yes. Did this particular play prove that an entire team can make a mistake? Yes, yes it did - the entire team did have a clear upper hand, K4tarina smashed their faces in for a small mistake in judgement.
One person making a mistake doesn't throw a game. An entire team making a mistake like that would, if they weren't so far ahead.
If you were truly ahead in every way, you'd have been able to capitalize on the win as the team did in the above video, and that's only mid-gold.