Re:Decline Relative To Whom?

The up-and-coming countries are China, India, and possibly Brazil

Agreed.

However the up-and-coming ones are growing at unsustainable rates.

In the case of India, Brazil, it might be so. But China has been showing rapid, steady, sustainable rate of growth in economy. China is an extremely big country. And it is surprising to see such a big country shows such a double-digit figure in economic growth rate.

The only question I have is whether they will "crash" or..

Neither of these countries won't crash but Japan will. Soon -within 2 or 3 years, this country will be gulped from USA, or being already gulped?

Decline relative to whom?

Particularly relative to China, Korea. China is sure to surpass Japan in every respect soon -within 10 years.