One country's decline is a benefit to another one. It might sound cold and selfish but the truth is that if one economy is declining, you can use this to your advantage. If one country has a failing economy and they can no longer produce high quality products and their trade is losing significance then another country can sweep in and give better offers. This is simply the reality and so economy should not have emotions in because it will not do you good. If you want to improve your financial state, you have to compete. This is true even with smaller scale marketplace. Not a country's but let's say in one city. If one seller fails to deliver good products, you can sell better ones and take their customers. There is no space in the economy for soft hearted because everything is a competition. You have to compete for resources. You have to compete for demand. You have to compete for customers.
Economically the world is a global market. Just as businesses are competing for market share, nations also try to outsmart each other. The competition is so fierce that it could lead to attacks to sabotage economies. China became the second biggest economy because it took advantage of its population and technological development to take over the market share of many countries. With cheap labour and fast production speed China was able to produce cheap quality products. Many countries had to diversify into other areas while they depended on China for diverse products.