Reason for Economic Slowdown in Western Nations Why do the U.S and other major Western Countries (e.g, Canada, Germany, in contrast to India, Australia, and China) have stagnated GDP growth? Comparing the GDP growth in the post-2000s from the 1990s and previous decades, the trendline seems to go down. Government debts have also increased by a lot among many countries. Is there a reason applicable to all of these nations that explains this?
The primary reason is that it is easier to catch up than it is to forge ahead.  India and China have lower GDPs per capita, so they can copy what other countries have done and gain.  But the United States, Canada, and Germany can't copy from India or China, as they are already higher.  Copying would make them lower.  And there's no one higher from whom they can copy.  They can copy a little from each other, but not enough to maintain high growth.  

A good example is Japan.  Japan used to experience fast GDP growth.  Then it caught up to countries like the US and Germany.  Since then, it has stagnated.  

Presumably this will also happen with China and India, but their high populations mean that it might not happen until their GDPs are bigger than those of the United States.  

Note that China remains below the world average in GDP per capita.  Presumably it can grow until it is closer to the US or Germany.  

Of course, it is also possible that there is something about the Chinese system that will limit it before then.  We can only speculate until their GDP stabilizes.  Some have speculated that China's growth is stabilizing now (it's been decreasing steadily since 2010), but we will need more time to make that determination.  At least ten years after it hits a steady rate, which it has yet to do.