The main thing that's exercising me is that if 1WG is an impossible/undesirable ideal, what future does humanity have?
It may well have very long future. Notice that levels of violence worldwide have been falling since WWII. It may not seem that way because violence gets
reported more, but fewer people were killed in 2006-2010 than in 2001-2005 -- and fewer in 2001-2005 than in 1996-2000. And number of independent states went up during the same time.
This is very ironic considering what science fiction writers had imagined for decades. Throughout 20th Century SF assumed that any prosperous future would by necessity involve unified humanity -- that divided states would
always fight, and only unification enables peace. What happened is the inverse -- peace enables disunity. It was difficult to be a small country in 1900. Small internal market condemned population to poverty. Covetous neighbors might attack. And there was always a danger that some sergeant would storm the royal palace and declare himself king.
Whereas nowadays being a small country is much easier. If you produce something valuable, global trade opens a big market. However much better it could be, international community does MUCH better job than in 1900 keeping bullies in check. And palace coups and such are more difficult in the age of ubiquitous phone cameras and Internet. So there is little pressure for unification -- quite opposite.
All through history wars and other calamities had forced diverse tribes to unite into larger groups. Today peace and prosperity make it attractive for large groups to split back into (mutually more-or-less friendly) tribes.
[Edited] Now that I think of it, if the more dire predictions of Paul Ehrich and Martin Reese come true, and environmental degradation makes world a nasty place again, conflicts will go up --
and provide a push back to unification. Not necessarily all the way to 1WG, but perhaps to regional hegemonies.
Which makes for an interesting answer to OP -- World Government is not necessarily a bad thing in itself, but it takes very bad things to bring it about! Which is not at all desirable.