By the time it becomes a real problem (when the db containing state starts to approach 4TB) presumably some other solution would be in place, like state witness

By the time it becomes a real problem (when the db containing state starts to approach 4TB) presumably some other solution would be in place, like state witnesses in blocks (verification without needing to access local state), or other ideas like reducing the size of 'hot' state and requiring witnesses for old state. In any case, it's better to have ethereum with heavier nodes than everything move to a completely centralized DPoS or PoA network.