So What Stops Apple from hiring the ARM Team and design the next generation of big.LITTLE independent of ARM, without actually purchasing ARM the company?

If reports are correct they did hire some of the Imagination GPU design team.

So they probably could but whilst the relationship is good the best of all worlds is a strong ARM team plus a strong team in-house.

Actually Apple benefits if the ARM Core team isn't very good, the rest of the industry would suffer if Mali GPU's dont get upgrades. Already they are behind iPhones, a poor team at ARM Ensures competition doesn't catchup. But all this might change should NVIDIA buys it and adds their own GPUS to the SOC.

Lots of existing GPU alternatives already - including Nvidia (Tegra) so can't see this as a game changer.

Nvidia Tegra is a rent seeking expensive chip. Nvidia has a habit of stopping people from using certain chips through EULA. None of the car manufacturers are using Nvidia Tegra in their autonomous drives for this reason. Even Tesla which started out with Tegra stopped using it.