That's the thing.
If you are 4 manned the whole first 15 minutes of the game and your jungle/bot have lost the first 15 minutes, and you say, ONE TIME, "We are going to lose", or, "We can't win this," I myself do not consider this toxic behavior, especially if you are analyzing the current game and you understand what it takes to win.
I agree, it is not the best thing to say, but that's the WONDERFUL thing about LoL, you can't type an essay explaining why you think the team will lose the game, because then a multitude of things can happen - example you will be banned for spamming, etc, and your team will think you are spending more time typing than playing.
I have played this game since season 1. Do you know what I have found out? 9/10 games will have someone in it, who is doing lackluster and not their best, and will speak out about it. It's called being human.
What I think is a bigger problem, is when a neutral comment is made, and it is interpreted another way by another person of a different INTELLIGENCE level. And there it is, the elephant in the room, yes, intelligence, comprehension, critical thinking and problem solving. All go hand in hand when analyzing something and making an educated guess.
If my team was overwhelmed by the enemy team, and we were all in unanimous agreement that the other team obviously was more coordinated, and accepted that we may not win, is that not MORE constructive than arguing with the person having a bad game and making things worse?
Just some thoughts from a long time LoL player.