Really? So, you believe that physics is an overly fragile structure? Or chemistry? Or mathematics? Those are all systems governed by an ideal order.
These are just constructs of the human mind, and would never exist in this way were humans not in conflict with everything around them. If all of humanity were to be in agreement, these ideas, and really all other ideas, would have stagnated long ago, with no maverics to promote outlandish ideas (quantum physics, relativity, etc). However, in your ideal universe humans have already perfected these ideas (somehow) so I'll drop that point.
If anything, a single, united humanity would be far more capable of averting problems, as everyone would know everything.
But that's totally impossible, even as an ideal! There always will be a sphere of information beyond our perceived universe. A vast unknown out beyond the visible light barrier! Paranoids and crazies would protect us/explore this region, but a complacent oneness would compute the probabilities and decide that there was nothing to be done.
The bottom line is, to me, that the universe is far too unpredictable (look at quantum theory, doesn't that suggest an unfathomable randomness to the universe?) for a single entity to survive. Inidividuality provides security by fighting the chaos with more chaos, a single being would inevitably succumb to the entropy that surrounded it.
I need to sleep.
I eat pasta!