Tom:
If it was just about economics, I'd say India (with a middle class currently numbering 350 million, and growing) followed by Brazil (staples producer for the world, and with investment already flowing outward). China I think will run into economic/demographic problems: so far their economy has kept growing because of the migration of the rural population to the cities, but that can't go on indefinitely.
But of course it isn't just about economics. I remember years ago, just after the Soviet Union collapsed, there was a very wise fellow who predicted that of all the countries formerly under the sphere of Soviet influence, the one that would do the best was the Czech Republic. He's been proven right, and I think he was right about the reasons as well.
He said that the Czech Republic was one of the few countries there that had a 'democratic infrastructure', i.e. most of the other countries, even before the Soviets, had been under one form of dictatorship or another for decades and centuries (i.e. many generations). They could be granted all the freedom possible, but still wouldn't know what to do with it, not for many years to come; democracy simply wasn't bred in their bones. The Czechs, on the other hand, had known and lived democracy for many years prior to the Soviet's coming, and sure enough were the first to bounce back, politically and economically. So look for the 'infrastructure' for a clue.
Sorry, I can't take the argument any further and answer your question; I don't know enough about the 'sporting/golfing infrastructure' of some potential golfing countries. So in the end, I'd probably guess the U.S. will still lead the world.
But when you ask "Will the USA still be devoting more of its resources and good land to golf?", I think that's a very different question. To that question my guess would be 'no', that in 50 years the "good land" won't be going to build golf courses in the U.S.
Which, by the way, is one of the (secondary) reasons I think that Ballyneal will keep steadily rising up the rankings list; I mean, really rising. It's another guess, of course: I have only read about and seen pictures of Ballyneal. But for what it's worth, I have a feeling a course like Ballyneal is going to be giving the top 1 and 2 courses in the U.S. a real run for their money in years to come. It'll have something to do with nature, and naturalism.
Peter