I can appreciate the premise a lot.
We can nitpick about this or that, whether Garreau's treatment is accurate or not (or whether it was in/accurate even when the book was published), but it doesn't change the overall message: you are where you live.
The fundamental fault with nation-states - that we all instinctively acknowledge - is just this: they are artificial. Differences might be eroding over time, but not necessarily. On the contrary: we have Scotts who want to be Scottish not British, Quebecers who want to be Quebecers rather than Canadian, the Basques, the Sami, etc. The list is getting longer - not shorter.
In moral stand point alone we should always let "nations" (for a lack of better word) be what they want to be. The geography matters. It always has and always will.
For example New Yorkers aren't exactly Americans either. They just happen to speak English (or at least try to for a whopping 36% of New Yorkers (and that particular piece of statistics is outdated by 8 years already, mind you) are foreign-born - and the rest are either out-of-towners or just plain old, largely anonymous suburbanites). New Yorkers are Big City Dwellers. Just like those living in Moscow, Tokio, Paris, London and so on. They have a different lifestyle, a different mind frame than someone who for example lives in a small town - let alone village or a farm. Just like the one who lives in Ithaca has less to do with him/her being an "American" than it is about him/her being someone who - for one reason or the other - resides in a distinctly University Town. Call them U-Towners if you will.
A yuppie (are we still allowed to use that term?) New Yorker can't expect to stay the same if s/he is to move to a less crowded place. Ditto with rednecks. It wouldn't work, it never does.
I'm guessing before patriotism people just got along. As neighbors they pretty much have to. They might hate each others guts but what can they do? If neither one is willing to call it quits and leave, there's only one thing they can do without a fear of retaliation: you learn to tolerate. In time you might learn to see your neighbor's point of view - or at least your kids will. Sometimes it takes a generation or two to dispel idiocy. In the end they all get along famously. And all is good.
That is until some first-grade assholes come along, tell you lies about your neighbor and finally ask you to wipe them off the face of the planet - for no good reason... Well, other than the gun pointed at your own head. We hate to kill but we dislike dying even more. It's just that simple.