This reduces player activity and diminishes interest in the forum as a whole.
There will be fewer reports to read.

What's the reasoning behind this? If someone wants to write a confirmation (even if it's no longer mandatory), explain the reasons for their defeat, or talk about their game, they will. Mandatory posts like "confirming, thank you for the game" are unlikely to significantly increase player activity or interest in the forum.

If someone doesn't confirm, meaning they disagree, they should write (not confirming because:)

Exactly. If the mandatory confirmation is removed, they will have to do this; simply staying silent will no longer be an option.

I'm amazed by people who, after spending 10 hours on a game, can write only 2-3 words in their confirmation. And you want to remove even that!)

If they don't write, it means they don't want to; maybe we shouldn't force them? ;)

What is the purpose of all this? If it's for quickly calculating the rating, it's already updated once a month anyway.

For example, to make the job of the rating administrator easier, and for the convenience of the players.