Green is a 'genuine' all-rounder, not a batting or bowling all-rounder. Andrew Flintoff was a genuine all-rounder who had a celebrated career (perhaps a bit over-rated, truth be told, but he was very good). Flintoff averaged 32.78 with the ball and 31.77 with the bat. Green's sample size is still small, but he averages 27.37 with the ball and 30.58 with the bat. By that comparison, Green's start has been a raging success.
Ben Stokes is a batting all-rounder, and he averages 32.11 with the ball and 36.12 with the bat. What Green already offers across both bowling and batting is comparable to those numbers. Of course, Green does not have 'talismanic' powers, and he has only played eight tests, so it's too early to judge. But there is more than enough there for the selectors to stick with him.
hopefully this time next year, he'll have a test ton under his belt and a 5fa.