Amla no doubt incredible batsman and an ATG. But given the way times are changing now-a-days, if you have openers with s/r 90 and avg around 40, you still can be a top team. But it is mandatory to have good middle order and lower batsman/finishers now a days. That is why i feel Amla's superior 'Average' is a bonus for South Africa, not absolute necessary. But having finishers like de-villers/Dhoni is absolute necessary. Having a chaser and a crunch player like Virat Kohli is absolute necessary. If south Africa have to chose between Amla and De-villers, they would probably chose devillers. If (hypothetically)South-Africa will give a chance to India to trade kohli for Amla, they would probably never do.
For me De-villers>Kohli>Dhoni>Amla and rest others.