Any deep discussion on this will likely get kneecapped by the mods as political. I like Steyn as he tends to walk the edge and provoke thought. I happen to think he's right on this one.
Japan S. Korea might be included too IMO. And some Latin American countries excluded.
My bet is that within 1 generation same would happen to more developed Islamic countries - old Roman proverbial about bread games seems very applicable after almost 1400 years...
The declines that Styn has highlighted - UK and USA (Canada by default) - are both Christian societies with strong sense of "fair play" and tolerance. It is this that as much as anything else has allowed Muslim culture to infiltrate into broad social values.
France is equally vulnerable and Germany is waking up. ("Multi-culturalisn doesn't work" -- Angela Merkle)
Korea and Japan while highly affluent are very "tribal" about their culture - Korea is one of the most racially "pure" (if you will forgive that anachronistic term) of all societies.
Korea and Japan may suffer from excesses of bread and circuses but will not lose their identities the way Great Britain has. Obviously - with debt comes vulnerability - and that is the major point Styne is making about the US, I think.