I think we can agree on everything only in the far future, when "we" are hyper-intelligent entities. Much like in the Culture of Iain M. Banks' novels - the minds in that universe do not agree on everything, but imagine them being advanced a bit further. Eventually you reach a stage where the knowledge attained by each being is so great that being might as well be a God-like being that's all-knowing.
If a collection of all-knowing beings exists, they may very well all agree on everything, because they know everything. I doubt we'll ever "evolve" or improve ourselves to such a stage, but theoretically speaking, I think this is the only scenario I can think of in which a collection of 6 billion minds would agree to everything. Unless they were all brainwashed or something, but that seems like a cop-out.
I also agree that without disagreements there would be no progress. That seems so obvious in hindsight, but I wouldn't have thought to say that, I guess probably because I'm not a philosopher.