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.
Decline isn't always the case though. I will like to use US for example, most demand of trades comes from the US, even China, Canada, Indian can't do without them and that's why they were trying so hard to bend for Trump the other day when he imposed increased in trade tarrifs to them. If today US economy is affected, it's going to affect other countries that highly depend on them for sales. If India has a rough economy on their pharmaceutical industry, the rest of the African countries are going to suffer because that's the only place they get the cheapest medicines.
The only time an economy decline favour's one when two countries are in competition of exportation, let's say China is number producer of silk materials and are well known for it and Germany also produce silk materials but all of a sudden Chinese is hit by an outbreak that made the country to lock down for long time, Germany is going to have the opportunity to make good sales to other demanding countries boosting their economy while China suffer a decline in sales.