[2019] Why is the "Decline" button literally useless?

It's because of this reason. You are also likely to hit the queue pop with the same people because of how matchmaking works (you're all similar skill, you've been waiting long enough to be at the front of the queue, and if you your lobby gets dodged we try to get you a match more quickly). So it makes sense to get all AFKs out of queue in one go because it's likely you'll just encounter them on the next queue pop.

I advocated for this when we were originally designing the system, and implementation concerns were serious enough that we just went with the simpler behavior of keeping everyone in AFK check till everyone accepts/declines/times out. The hazard is that our queue times are so low in popular queues that your queue could pop in a new match before the old AFK check expires which would necessitate a lot of logic to handle that correctly and higher risk of bugs that could be hard to find and fix. That part of the game flow was never designed to pull individuals in or out of a match.