The problem with all-chat in League of Legends, particularly in the ranked queue, is that it can only hurt your team's chances to win. At best, you're wasting time typing; at worst, you're actively frustrating and distracting yourself from what is already a very difficult game.
All-chat, as some have already noted, is actually an opt-in feature already; you have to turn it on. I've had it turned off for about two years. I don't like it; it makes me play worse. That is beside the point.
In my opinion, the point of this change (from Riot's perspective) is not to shield you, individually, from your opponents' flame. /mute exists, and if you want, you can turn /all off entirely. What you can’t do is turn it off for your teammates, and that’s the problem.
The point of this decision is to stop people from engaging. And for that, I am grateful. I can count many, many games that I knew my team could win, if only they would stop getting tilted and typing back at what the enemy team was yelling at them in /all. All I want -- and apparently, all Riot wants -- is for people to just stop yelling and play a gosh darn videogame.
The complaint, as far as I can tell, is that you lose beneficial interactions from this, too; "glhf [good luck, have fun]" at the start of the game, or "nice play" from someone you just killed.
I have played a lot of this game; comfortably thousands of hours. I understand that this is anecdata from some guy on the internet, but I feel extremely confident in saying that the positive interactions are a minority; even if they're not, I would be hard-pressed to say that they outweigh the games that I've lost, or the anger/frustration I have seen teammates feel, as a result of /all.