I've always wondered about this. Tanks are essentially mobile pillboxes/strong points. 2000 years ago the chariot was the tank of the Assyrians and the Egyptians only to be made obsolete by cavalry and changing infantry tactics. It reappears again and again in some form or another. A tough question. I do not think that it will ever be replaced, only limited for a time and then taken out of mothballs or reinvented. Could I be wrong?