The point is there are already enough GS brackets at the moment, it's just that if there are not enough people in queue with similar GS, you get match against random GS people in bgs. To enlighten, you cannot always get queued 100% with similar GS people of your bracket if there are not enough people of similar GS like yours that are in queue.
Well, joining bgs right after hitting 80 with 12k hp is like a person with disability trying to swim, and that will eventually result in that disabled person drowning. With 12k hp you will be of no use to your team either with dps, heal, or with objectives because you will have to spend at least few seconds somewhere to get the objective done, so by then you can easily get one shot out of anywhere by anyone.