It is not possible for networks to "diverge to the point of no return" because they can always drop their divergent chain for the longest one.
Sure, but the issue is that the chains might have diverged so much that to abandon the chain with less proof of work would mean reversing weeks or months or payments, wiping thousands of bitcoin completely out of existence due to lost block rewards, and completely invalidating every transaction which has one of these lost block rewards anywhere in its history. The combined chaos of all that happening might mean that the people using the minority chain deliberately make changes to their protocol to prevent their nodes from abandoning their chain in the event of communications being reestablished, leaving us with two networks running side by side.
As I said before if the global internet suffered such an outage for such a long period of time we are going to be living in a MadMax / World War Z kind of scenario so at least for the few of us that are still alive it's not going to matter much.
Interesting enough, in the event of a global internet outage big cities and suburbia will probably have a much better time of it then the middle of nowhere. With most of the large data centers in populated areas it would be easier for them to get access back to the people close to them, then someone living in the middle of a farm 200 miles from nowhere.
-Dave