Earlier quoted context omitted.
This is an interesting scenario worth thinking through. If Bitcoin is forced to go split-brain temporarily, the network could coordinate to rejoin after the fact. Or, it could stay split in two. Regardless, infighting over which network was “the real Bitcoin” would ensue. See Bitcoin Cash and friends.
A "rejoin" isn't really possible. People who spent money on the "wrong" chain will simply have to live with the fact that their money is lost on the "right" chain, although it will probably not be an overnight thing. The two forks could exist for a long time after the split simultaneously, and many merchants may accept either form of payment. There will even be arbitrage between the two chains as profit can be made.…
It's not that simple. Transactions can, and will be, replayed on both chains. There would however be many opportunities for attacks such as double spends for a short window of time.
> The two forks could exist for a long time after the split simultaneously,
That's not realistic. As soon as there is some communications between the chains, the system will reconcile. It's sort of the whole reason why this proof-of-work thing exists. Without the need to reconcile chain splits it would be sufficient with some sort of voting of proof-of-stake scheme.