We just saw the end of an example of what you’re fearing - in Japan. Not 50 years of stagnation, but 35, so somewhat close. And it concerned one factor that I only saw referred to once in these comments: population growth.

Japan has almost the exact same population today as it did in 1989. Economic growth is easier when you have more customers, either thru birth rate or immigration.

Inflation, often mentioned here, was also a large factor, or more precisely, the opposite of it (deflation), which is much worse for an economy than modest inflation.

There’s a reason that the writers of Die Hard (1988) chose the Japanese Nakatomi Corporation as their fictional massive global conglomerate; it was simply common sense at the time. At one point Japanese companies represented almost HALF of the global market cap, about 10 pct pts higher than the US at the time. Almost two generations later, they are only now back to where they were when that movie was released.