The space of playing go is rather large and humans only need to be better at a small portion of it to improve upon AlphaGo. Humans+Computer was better than just Computer for chess (at long time controls) for nearly 10 years after BigBlue beat Kasparov.
That is for a recent tournament. Now computers are just better. But for up to 5-10 years centaurs were better. Eventually human Go players won't be able to improve on computer evaluations, but that is may not be for another few years.
