If we branch now, we run a serious risk that all these bugs will be fixed twice. We saw that with the very long-running 4.6 branch.
And merging is painful as well, because it's hard to keep a clear state of what's been merged and what hasn't been merged. A simple efficient strategy is to merge each release back to the branch, but that means that the branch and HEAD diverges inbetween releases (with duplicate fixes for the same bugs).
I'd much rather we spend a few weeks shaking out bugs, then branch.