That cannot happen. Cities need a lot of "empty" space around them for farms. Well I suppose everyone could move to Spain (picking a random country) and leave the rest of the world to robotic farms. However we can't expand to cover even all of Spain unless everyone is living in a mega suburb with everyone having their own single story mansion.

(others have already pointed out that populations are on track to fall shortly and there is no reason to think that trend will reverse though nobody knows)